You can have an enterprise release of the app i.e add the udid of all the people's devices you want to give access through and distribute the app through private network. The number has a limit, 100. You might need more then one account if number is large.
EDIT
Enterprise Distribution (Enterprise Program, $299 / Must have D-U-N-S number)
The iOS Developer Enterprise Program is intended for companies who create proprietary, in-house iOS apps for internal deployment within the company only. A valid Dun & Bradstreet (D-U-N-S) number is required for program enrollment.
Ad Hoc Distribution (Available in Standard Program, $99)
The Standard and Enterprise Programs allow you to share your application with up to 100 other iPhone or iPod touch users with Ad Hoc distribution. Share your application through email or by posting it to a web site or server.
if you are registered with enterprise development program with apple you can do it link