Hi malie22001,
There are a few ways to uniquely identify a Membership user. The official recommendation by Microsoft is to use a combination of the UserName and the application name (which you set in the web.config file on the Membership and other configuration elements). You should store both in your custom tables.
As an alternative, you can store the user's unique ID, a GUID in the case of the standard SqlMembershipProvider, which you can get by calling Membership.GetUser().
ProviderUserKey.
Microsoft recomends against using the ProviderUserKey as its internal implementaton which *might* change in the future. However, it hasn't changed since the first release of Membership so you're probably safe if you use it.
Cheers,
Imar