Invite-based registration system for Multisite/BuddyPress

Keen to hear your suggestions on the best solutions for managing registration and sign-up for MU/BP site so that a visitor can;

a) replace the standard WP sign-up
b) users register (using just their email address) to be invited to join the community
b) admin can manually send out invite codes to selected users
c) users are then able to use the invite code to complete the registration process as normal.

Thanks