FEATURE: Group logs.

app/models/group.rb

@@ -9,6 +9,7 @@ class Group < ActiveRecord::Base
 
   has_many :categories, through: :category_groups
   has_many :users, through: :group_users
+  has_many :group_histories, dependent: :destroy
 
   has_and_belongs_to_many :web_hooks
 
@@ -347,7 +348,8 @@ class Group < ActiveRecord::Base
   end
 
   def add(user)
-    self.users.push(user)
+    self.users.push(user) unless self.users.include?(user)
+    self
   end
 
   def remove(user)