"humm... Besides 'Host' users and a possible 3rd party trick, users are 'per portal'. A UserId is not shared among portals."
I just tested this and the UserId appears to be shared across the portals.
Logged in as Host
Added user to portal 1
Moved to portal 2
Added exact same user information
Went to the Edit User for both portals
Admin/UserAccounts/tabid/227/ctl/Edit/mid/622/UserId/3/filter/All/currentpage/1/Default.aspx
Admin/UserAccounts/tabid/45/ctl/Edit/mid/358/UserId/3/filter/All/currentpage/1/Default.aspx
I went to check my Users and UserPortals tables, and apparently I had entered this test user on portal before, that I forgot about. So I also have
/Admin/UserAccounts/tabid/125/ctl/Edit/mid/469/UserId/3/filter/All/currentpage/1/Default.aspx
There is one entry in the Users table for this account (actually the only non-default account).
There are 3 entries in the UserPortals table for UserID 3, corresponding to the 3 portalIDs that I put him in.
EDIT - When I deleted the user from 1 portal, it changed that IsDeleted flag in the UserPortals table, but not the Users table.