Are you going to let just anyone who can open the db become a user? If so, you ought to have an AutoExec macro determine if they are already registered (people will probably re-register simply because they forgot their password). If they are not, present the sign up form. Have a button on the form with a click event to save the record. If they are, then don't open the sign up form but provide a starting point from which to interact with the db - maybe a switchboard form. I would not use a navigation form since so many people seem to post about issues when trying to get them to work. BTW, the way you have it, I will be able to see anyone's sign up information by cycling through the records.
Personally, I would ditch the idea altogether. An admin person should be entering user credentials into the user table, and if the startup routine doesn't find them, they don't get in. This means passwords are not required, thus they are not vulnerable to exploitation. As long as you use a bit of forethought, you should be able to keep most people out who shouldn't be getting in (such as hiding the nav pane, disabling keyboard shortcuts, disallowing the shift by-pass) though an Access db will never be 100% bullet proof for some people.
EDIT: as an after thought - your current process will also require you to implement a password changing and/or recovery feature, which is another whole chunk of design work. If you choose not to bind the controls of a sign up form (as you currently have designed) this becomes an ever bigger task.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.