gistfile1.rb

class Group < AR::Base
  has_many :memberships
  has_many :members, through: :memberships, class_name: "User"
  has_many :global_admin_members, through: :memberships, class_name: "User", conditions: { admin: true }
end

class User < AR::Base
  has_many :memberships
  has_many :groups, through: :memberships
end

class Membership < AR::Base
  belongs_to :user
  belongs_to :group
end