Pre Registration Form Validation Help

Hi,

My network is about to be launched in the month of Jan 2015.

I wanted to have a Pre registration Session to closed user group.

I will describe how I want it to be.

1. Users Fills the form details (Same fields as required for creating a blog in a multisite)

2. Here is the problem. When User 1 registered his domain as user1.mysite.com this should not be available to others right. How will I let know the user 2 who will use the same form?

It would be great if a user is notified on the front end after he clicks submit button May be like this "SELECTED DOMAIN IS ALREADY BOOKED. PLEASE TRY ANOTHER"
(Now he will select another one and completes the registration.)

After that I would upload the list of Pre registered sites on the launch date.

Some kind of conditional logic should be done I think so.

Any Quiz plugin or survey plugin or contact form plugin can do this job with little coding help form your side.

NOTE:- Pre registration is not done with signup.php instead with a form plugin on the front end.Since Users will be allowed to their dashboard only after launch date

Please Help me, since I had to suddenly present my site to a audience of 300+ on 19-12-2014.

Any help within 19-12-2014 will help my business to grow since it is a one time chance for me. I have no one to ask help right now except You (WPMU)

Sincerely,

Thanks for taking time to address my problem.