diff --git a/app/assets/javascripts/groups.js.coffee b/app/assets/javascripts/groups.js.coffee
new file mode 100644
diff --git a/app/assets/javascripts/testcases.js.coffee b/app/assets/javascripts/testcases.js.coffee
new file mode 100644
diff --git a/app/assets/stylesheets/groups.css.scss b/app/assets/stylesheets/groups.css.scss
new file mode 100644
diff --git a/app/assets/stylesheets/testcases.css.scss b/app/assets/stylesheets/testcases.css.scss
new file mode 100644
diff --git a/app/controllers/groups_controller.rb b/app/controllers/groups_controller.rb
--- a/app/controllers/groups_controller.rb
+++ b/app/controllers/groups_controller.rb
@@ -52,25 +52,33 @@
   def remove_user
     user = User.find(params[:user_id])
     @group.users.delete(user)
-    redirect_to group_path(@group), notice: "User #{user.login} was removed from the group #{@group.name}"
+    redirect_to group_path(@group), flash: {success: "User #{user.login} was removed from the group #{@group.name}"}
   end
 
   def add_user
     user = User.find(params[:user_id])
-    @group.users << user
-    redirect_to group_path(@group), notice: "User #{user.login} was add to the group #{@group.name}"
+    begin
+      @group.users << user
+      redirect_to group_path(@group), flash: { success: "User #{user.login} was add to the group #{@group.name}"}
+    rescue => e
+      redirect_to group_path(@group), alert: e.message
+    end
   end
 
   def remove_problem
     problem = Problem.find(params[:problem_id])
     @group.problems.delete(problem)
-    redirect_to group_path(@group), notice: "Problem #{problem.name} was removed from the group #{@group.name}"
+    redirect_to group_path(@group), flash: {success: "Problem #{problem.name} was removed from the group #{@group.name}" }
   end
 
   def add_problem
     problem = Problem.find(params[:problem_id])
-    @group.problems << problem
-    redirect_to group_path(@group), notice: "Problem #{problem.name} was add to the group #{@group.name}"
+    begin
+      @group.problems << problem
+      redirect_to group_path(@group), flash: {success: "Problem #{problem.name} was add to the group #{@group.name}" }
+    rescue => e
+      redirect_to group_path(@group), alert: e.message
+    end
   end
 
   private
diff --git a/app/controllers/problems_controller.rb b/app/controllers/problems_controller.rb
--- a/app/controllers/problems_controller.rb
+++ b/app/controllers/problems_controller.rb
@@ -197,10 +197,20 @@
       set_available(false)
     elsif params.has_key? 'add_group'
       group = Group.find(params[:group_id])
+      ok = []
+      failed = []
       get_problems_from_params.each do |p|
-        group.problems << p
+        begin
+          group.problems << p
+          ok << p.full_name
+        rescue => e
+          failed << p.full_name
+        end
       end
+      flash[:success] = "The following problems are added to the group #{group.name}: " + ok.join(', ') if ok.count > 0
+      flash[:alert] = "The following problems are already in the group #{group.name}: " + failed.join(', ') if failed.count > 0
     end
+
     redirect_to :action => 'manage'
   end
 
diff --git a/app/controllers/user_admin_controller.rb b/app/controllers/user_admin_controller.rb
--- a/app/controllers/user_admin_controller.rb
+++ b/app/controllers/user_admin_controller.rb
@@ -442,7 +442,18 @@
       end
       if @action[:add_group] and @action[:group_name]
         @group = Group.find(@action[:group_name])
-        @users.each { |user|  @group.users << user }
+        ok = []
+        failed = []
+        @users.each do |user|  
+          begin
+            @group.users << user
+            ok << user.login
+          rescue => e
+            failed << user.login
+          end
+        end
+        flash[:success] = "The following users are added to the 'group #{@group.name}': " + ok.join(', ') if ok.count > 0
+        flash[:alert] = "The following users are already in the 'group #{@group.name}': " + failed.join(', ') if failed.count > 0
       end
     end
   end
diff --git a/app/helpers/application_helper.rb b/app/helpers/application_helper.rb
--- a/app/helpers/application_helper.rb
+++ b/app/helpers/application_helper.rb
@@ -200,7 +200,7 @@
   BOOTSTRAP_FLASH_MSG = {
     success: 'alert-success',
     error: 'alert-danger',
-    alert: 'alert-block',
+    alert: 'alert-danger',
     notice: 'alert-info'
   }
 
diff --git a/app/models/group.rb b/app/models/group.rb
--- a/app/models/group.rb
+++ b/app/models/group.rb
@@ -1,5 +1,13 @@
 class Group < ActiveRecord::Base
-  has_and_belongs_to_many :problems
-  has_and_belongs_to_many :users
+  has_many :groups_problems, class_name: GroupProblem
+  has_many :problems, :through => :groups_problems
+
+  has_many :groups_users, class_name: GroupUser
+  has_many :users, :through => :groups_users
+
+  #has_and_belongs_to_many :problems
+  #has_and_belongs_to_many :users
+
+
 end
 
diff --git a/app/models/group_problem.rb b/app/models/group_problem.rb
new file mode 100644
--- /dev/null
+++ b/app/models/group_problem.rb
@@ -0,0 +1,7 @@
+class GroupProblem < ActiveRecord::Base
+  self.table_name = 'groups_problems'
+  
+  belongs_to :problem
+  belongs_to :group
+  validates_uniqueness_of :problem_id, scope: :group_id, message: ->(object, data) { "'#{Problem.find(data[:value]).full_name}' is already in the group" }
+end
diff --git a/app/models/group_user.rb b/app/models/group_user.rb
new file mode 100644
--- /dev/null
+++ b/app/models/group_user.rb
@@ -0,0 +1,7 @@
+class GroupUser < ActiveRecord::Base
+  self.table_name = 'groups_users'
+  
+  belongs_to :user
+  belongs_to :group
+  validates_uniqueness_of :user_id, scope: :group_id, message: ->(object, data) { "'#{User.find(data[:value]).full_name}' is already in the group" }
+end
diff --git a/app/models/problem.rb b/app/models/problem.rb
--- a/app/models/problem.rb
+++ b/app/models/problem.rb
@@ -2,7 +2,11 @@
 
   belongs_to :description
   has_and_belongs_to_many :contests, :uniq => true
-  has_and_belongs_to_many :groups
+
+  #has_and_belongs_to_many :groups
+  has_many :groups_problems, class_name: GroupProblem
+  has_many :groups, :through => :groups_problems
+
   has_many :test_pairs, :dependent => :delete_all
   has_many :testcases, :dependent => :destroy
 
diff --git a/app/models/user.rb b/app/models/user.rb
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -7,7 +7,10 @@
 class User < ActiveRecord::Base
 
   has_and_belongs_to_many :roles
-  has_and_belongs_to_many :groups
+
+  #has_and_belongs_to_many :groups
+  has_many :groups_users, class_name: GroupUser
+  has_many :groups, :through => :groups_users
 
   has_many :test_requests, -> {order(submitted_at: DESC)}
 
diff --git a/app/views/problems/index.html.haml b/app/views/problems/index.html.haml
--- a/app/views/problems/index.html.haml
+++ b/app/views/problems/index.html.haml
@@ -1,10 +1,10 @@
 - content_for :head do
   = stylesheet_link_tag 'problems'
-%h1 Listing problems
+%h1 Problems
 %p
-  = link_to 'New problem', new_problem_path, class: 'btn btn-default btn-sm'
-  = link_to 'Manage problems', { action: 'manage'}, class: 'btn btn-default btn-sm'
-  = link_to 'Import problems', {:action => 'import'}, class: 'btn btn-default btn-sm'
+  = link_to 'Import problems', {:action => 'import'}, class: 'btn btn-success btn-sm'
+  = link_to 'New problem', new_problem_path, class: 'btn btn-success btn-sm'
+  = link_to 'Bulk Manage', { action: 'manage'}, class: 'btn btn-info btn-sm'
   = link_to 'Turn off all problems', {:action => 'turn_all_off'}, class: 'btn btn-default btn-sm'
   = link_to 'Turn on all problems', {:action => 'turn_all_on'}, class: 'btn btn-default btn-sm'
 .submitbox
diff --git a/app/views/problems/manage.html.haml b/app/views/problems/manage.html.haml
--- a/app/views/problems/manage.html.haml
+++ b/app/views/problems/manage.html.haml
@@ -36,33 +36,37 @@
 
 %h1 Manage problems
 
-%p= link_to '[Back to problem list]', :action => 'list'
+%p= link_to '[Back to problem list]', problems_path
 
 = form_tag :action=>'do_manage' do
-  .submitbox.panel
-    What do you want to do to the selected problem?
-    %br/
-    (You can shift-click to select a range of problems)
-    %ul
-      %li
-        Change date added to
-        = select_date Date.current, :prefix => 'date_added'
-        &nbsp;&nbsp;&nbsp;
-        = submit_tag 'Change', :name => 'change_date_added'
-      %li
-        Set available to
-        = submit_tag 'True', :name => 'enable_problem'
-        = submit_tag 'False', :name => 'disable_problem'
+  .panel.panel-primary
+    .panel-heading
+      Action
+    .panel-body
+      .submit-box
+        What do you want to do to the selected problem?
+        %br/
+        (You can shift-click to select a range of problems)
+        %ul
+          %li
+            Change date added to
+            = select_date Date.current, :prefix => 'date_added'
+            &nbsp;&nbsp;&nbsp;
+            = submit_tag 'Change', :name => 'change_date_added', class: 'btn btn-default'
+          %li
+            Set available to
+            = submit_tag 'True', :name => 'enable_problem', class: 'btn btn-default'
+            = submit_tag 'False', :name => 'disable_problem', class: 'btn btn-default'
 
-      - if GraderConfiguration.multicontests?
-        %li
-          Add to 
-          = select("contest","id",Contest.all.collect {|c| [c.title, c.id]})
-          = submit_tag 'Add', :name => 'add_to_contest'
-      %li
-        Add problems to group
-        = select_tag "group_id", options_from_collection_for_select( Group.all, 'id','name',params[:group_name]), id: 'group_name',class: 'select2'
-        = submit_tag 'Add', name: 'add_group'
+          - if GraderConfiguration.multicontests?
+            %li
+              Add to 
+              = select("contest","id",Contest.all.collect {|c| [c.title, c.id]})
+              = submit_tag 'Add', :name => 'add_to_contest', class: 'btn btn-default'
+          %li
+            Add problems to group
+            = select_tag "group_id", options_from_collection_for_select( Group.all, 'id','name',params[:group_name]), id: 'group_name',class: 'select2'
+            = submit_tag 'Add', name: 'add_group', class: 'btn btn-default'
           
 
   %table.table.table-hover
diff --git a/config/initializers/assets.rb b/config/initializers/assets.rb
new file mode 100644
--- /dev/null
+++ b/config/initializers/assets.rb
@@ -0,0 +1,23 @@
+# Be sure to restart your server when you modify this file.
+
+# Version of your assets, change this if you want to expire all your assets.
+Rails.application.config.assets.version = '1.0'
+
+# Add additional assets to the asset load path.
+# Rails.application.config.assets.paths << Emoji.images_path
+# Add Yarn node_modules folder to the asset load path.
+Rails.application.config.assets.paths << Rails.root.join('node_modules')
+Rails.application.config.assets.paths << Rails.root.join('vendor/assets/fonts')
+
+# Precompile additional assets.
+# application.js, application.css, and all non-JS/CSS in the app/assets
+# folder are already added.
+# Rails.application.config.assets.precompile += %w( admin.js admin.css )
+
+Rails.application.config.assets.precompile += ['announcement_refresh.js','effects.js','site_update.js']
+Rails.application.config.assets.precompile += ['local_jquery.js','tablesorter-theme.cafe.css']
+%w( announcements submissions configurations contests contest_management graders heartbeat 
+    login main messages problems report site sites sources tasks groups
+    test user_admin users ).each do |controller|
+  Rails.application.config.assets.precompile += ["#{controller}.js", "#{controller}.css"]
+end