You should know that if you made a login script. 😛
It would be easy. You use phpMyAdmin? Just create a new field, as siwis said, then when the user registers set it as 'guest' then you can use phpMyAdmin every day or something and simply edit the rows, and modify the data.
On the members pages where you check if they are logged in, you'd also check the database if that is guest, if it is, tell them they wait for you to activate their account.