i actually did the same thing for my client
and still working on improving it
(works fabilious)

make a table called USERS (to hold users)
make a global vriable called UserName
make another table called LoggedUsers
teach time a database/table is opened
get the username thats logged in
write it to the LOGGEDUSER table..
and read that table when checking

works GREAT
you got to do a lot of error checking
and make sure the user loggs off properly
ect...