On your landing page/app you can say something that "we select beta users with review/invitation bla bla we'll send you an invitation after the review bla bla please enter your email/phone bla bla"
After you get to a number of applications. Make it boom. Send mass mail/notif etc. They'll be in at the same time.
Some things to remember;
- Make sure you write something like beta/private so first users don't swear when they get in.. Or fake it with your fake users at first.
- Make sure the number of applications is enough to make it boom.