We implmented a concurrent user license scheme by creating a table in
the database to track active sessions. When a user logs on, the
application creates a record in this table. When he/she logs out we
delete the record. At periodic intervals while the application is
active it updates a time stamp on the session record. That way we can
reuse sessions that have a stale time stamp. This allows for sessions
that go away because the PC or network crashed or the user found some
other unique way to disconnect without logging off.

Not particularyly elegant but it works.