Well you have to be a Apple Developer, have to add the testers UDID (Use UDID from App Store) to the Ad Hoc distribution provisioning file profile. Do a Distribution build and then zip that up and email the user the provisioning file and the build.
lol! I think ibetatesters got a video tutorial on how to do that, or maybe check the faq: http://ibetatest.com/iphone/controllers/faq/