Confused about Multi-site and Users


I'm a bit confused about how I should best go about trying to configure multi-site / s2member pro/ with bbpress and my main site. I have bbpress set up as its own site within the network..but all my users still reside at main site. BBpress conflicts too much with my current theme to configure there. I should also add that I want my users to be able to upload their own avatars and not gravatars.

It seems if I let s2member handle all registration/login it wont allow for therefore i have defaulted everything back to wordpress..but of course I would like my users to access the network with 1 login not have to login separartely is this possible?