class User
     include Mongoid::Document
   ....
     belongs_to :invitee, :class_name => 'User'
     has_one :inviter, :class_name => 'User', :inverse_of => :invitee
   ...
   end

User.all.to_a.inspect =>
[#<User _id: 4e4a12d32e92d11f0d000001, _type: nil, email: "user@test.com", encrypted_password: "$2a$10$.GSwlja59LnMc9IqSBKAS.CRxJkUGoAoILYyF.LxLsdTx8I3p.586", remember_created_at: nil, reset_password_token: nil, reset_password_sent_at: nil, confirmation_token: "xFQ2afXhuaplgpHKxPMV", confirmed_at: nil, confirmation_sent_at: 2011-08-16 06:48:51 UTC, sign_in_count: 0, current_sign_in_at: nil, last_sign_in_at: nil, current_sign_in_ip: nil, last_sign_in_ip: nil, name: "Test User", gender: nil, date_of_birth: nil, admin: false, invited: true, invitee_id: BSON::ObjectId('4e4a12d32e92d11f0d000002')>, #<User _id: 4e4a12d32e92d11f0d000002, _type: nil, email: "user2@test.com", encrypted_password: "$2a$10$zHsp3aHMXGHIdxvu/94x.enpEyOjq.uby3mnKa45OJJqAEYZzLtCS", remember_created_at: nil, reset_password_token: nil, reset_password_sent_at: nil, confirmation_token: "x3tYAjZ41zzS4om3vIb0", confirmed_at: nil, confirmation_sent_at: 2011-08-16 06:48:51 UTC, sign_in_count: 0, current_sign_in_at: nil, last_sign_in_at: nil, current_sign_in_ip: nil, last_sign_in_ip: nil, name: "User2", gender: nil, date_of_birth: nil, admin: false, invited: true, invitee_id: BSON::ObjectId('4e4a12d32e92d11f0d000001')>]