There’s also Apple’s “Test Flight” program where you can upload a development/test version of your app to the app store for a minimal “App Store” review to be approved for testing.
Then you can add many users to your test release (external testers) using only their email address.
They would then download the “Test Flight” app on their phone which would allow them to download and run the test version of the app.
Once your test version is complete and ready for full release, you can just then promote the app you’ve already uploaded to production release (and a full app review from Apple).
You only need the $99/year developer account to get access to this.
Even though there are a few more hoops to jump through to get it working, I found this method significantly easier than getting all the users UDIDs and creating new provisioning profiles and compiling a new app version each time I wanted to add a test user for a release.