Are you aware of any sites that handle this issue?
PROBLEM: user experience during registration for a sub-site in a Multisite installation with a single BuddyPress social network. We want the Platform to be a unified social network. Sub-sites serve special purposes.
WHAT WE CAN DO:
If someone enters the Multisite platform by registering for the Primary site it is possible to have them join a BuddyPress Group and be registered for the Group Blog (Multisite sub-site) that has been associated with the Group.
THE BAD USER EXPERIENCE WE WANT TO IMPROVE:
If someone comes to the sub-site first and wants to register, they are taken to the registration form on the Primary site that has them fill out fields for the BuddyPress extended profile. At both that point and after authenticating by answering the email sent to them, they are in a different place (Primary Site) from where they started and there is no help for how to get back to where they started.
I have to believe others have solved this problem, but I have been unable to uncover solutions.