You can share membership across separate installs of Community Server. To do this, you create a main site and any number of satellite sites. Membership and profiles will be centralized, but each site will be responsible for its own roles. To begin, create at least two separate instances of Community...
In previous versions of Community Server, it was impossible to support shared membership with other Community Server sites or existing ASP.NET applications unless they were installed into a single database.
Shared Membership is the codename for the work we did across membership, profile, and roles in Community Server. A primary goal of Shared Membership was to allow sites to leverage existing membership stores in their implementation of Community Server. It was important to make it easy for multiple communities running Community Server to share a single membership store without their content being physically located in the same database.