Approach needed for a communities site

Hi!
I want to build a communities site, is gonna be built with WCF REST and Silverlight, but there are several ASP.NET things that I'd like to address about membership, roles and architecture.
More or less, the idea is this:
A user can connect to "http://mycomsite.com/" register itself, start a community and define its own users and roles. The user will do login once, and will be authenticated till he does logout or a timeout.The users can connect to the new community using "http://mycomsite.com/community_id/" and authenticate themselves. Again, each user will do login once, and will be authenticated till he does logout or a timeout. I can identify easily the community id through REST.
Each community will have its own database (this is a business requirement, alghough all2 will run in the same database server, but the database could be alocated in other server ). So the users and roles information for e ...