event.rb

class Event < ActiveRecord::Base
  belongs_to :user
  belongs_to :group
end

group.rb

class Group < ActiveRecord::Base
  belongs_to :user
  has_many :watchers
  has_many :members
  has_many :events
end

member.rb

class User < ActiveRecord::Base
  has_many :groups
  has_many :watchers
  has_many :members
  has_many :events
end

user.rb

class Member < ActiveRecord::Base
  belongs_to :group
  belongs_to :users
end

watcher.rb

class Watcher < ActiveRecord::Base
  belongs_to :group
  belongs_to :users
end