Actual invitation system is pretty useless. Who shoud use in 21th century email adress of friend to invite into website?
Making it more nowadays like can improve reach of each site running on BB.
What about referal link generator/QR code generator on each user profile? With it you can track and reward users for successful invitations. Link can be shared everywhere and makes more sense than email invitation system. Also I would appreciate option to make invitation with "auto join" some group after registration.