If you want to have them use the same database, but not use the same users,
take a look at the application features of the membership db. this let's you
create individual application areas for membership. Every user that
registers has an application id associated with their username which defines
the cope. In the membership provider settings in the web.config file you
specify the application name to validate against. this lets you use the same
exact provider and database in multiple web sites and only change the
application name in the provider.
Hope this helps,
Mark Fitzpatrick
Microsoft MVP - Expression