[eluser]Dam1an[/eluser]
Hi, the way I normally deal with this sort of thing is as follows
1) Make te first/last option in the drop down "Add new user"
2) When this is selected, it triggers an event via JS, which changes a hidden div below to be visible
3) The now visible div has the required fields for adding a new user, and is part of the same form
4) When the user submits the form, if the "Add new user" entry was selected, first create the new user and insert them into the database and get the ID (using db->insert_id()) and then carry on as normal
Or if it's an existing user, carry on as normal