Keep User Data base/Mutli Sites

I have been looking at post about multiple sites and it seems that with 3.0 everything changed. Basically what I have a root domain xxxxxxx.com that is currently running buddy press. I just got a new mobile template that I want to put at m.xxxxxxx.com but I don’t want my users to have to login again and I want to keep the same data. I see that with Multiple sites I can keep the “core” files but I need to access my members login to encase they are mobile. Any Ideas?