BuddyPress.org

Being a new user I have been playing around with Buddypress and managed to set up a Multiblog install which runs a number of subdomain installs, all using the default Buddypress-enabled theme (for now), as in:
site1.mydomain.com
site2.mydomain.com
site3.mydomain.com
(hint to new users: if you want a buddypress theme to work on all these subdomains, don’t forget to enable this line in wp-config:
define( ‘BP_ENABLE_MULTIBLOG’, true); It took me 2 days to figure that out)

Now, I am happy with the possibility of running different sites while sharing a community, however I’d like to differentiate between the users that signed up through these different sub-sites. For example, a user on site2 should be able to fully interact with a user who signed up on site1, BUT I’d like to have a way to list only the users that became members through either site 1, 2 or 3.
In that way, site1 would have a unique set of users that signed up through the site1 subdomain, and the same for the other subdomains.

If I understand correctly, there is currently no way to distinguish between users that signed up through these different subdomain sites?
Are there any plugins out there, or has anyone worked on making this possible? ‘Flagging’ a user so to say, so that they can always be filtered as being part of the ‘site1’ group of users. For the subsite admins this would offer the advantage of being able to choose between showing only members of their own site, or the members of the overall system (site1, 2 and 3 combined).