group.rb

class Group < ActiveRecord::Base
  has_many :group_memberships, :dependent => :destroy
  has_many :members, :through => :group_memberships, :source => :user
end

user.rb

class User < ActiveRecord::Base
  has_many :group_memberships, :dependent => :destroy
  has_many :groups, :through => :group_memberships

  # Include default devise modules. Others available are:
  # :token_authenticatable, :confirmable, :lockable and :timeoutable
  devise :invitable, :database_authenticatable, :registerable,
         :recoverable, :rememberable, :validatable
end