Actually when a user registers, only a small part of the data is stored globally.
As long as the user is not in the PortalUsers table, he will not be able to login to a portal.
So you can share a user between portals, but by default that's not the case.
The disadvantage of this is that as soon as a username is in use in one portal, you cannot use it in another one unless you enter exactly the same UN/PW combo.
In that case the user in the Users table will be used and a record added to the PortalUsers table.