When we talk about beta testing it is important to note that there are two different things that can be beta tested. The first is the companion app and that is done using Apple TestFlight. We've published links previously that allow anyone to join that beta program (open beta).
Regarding the build infrastructure for iOS, it originated as a closed beta. Due to a mistake on my part a few releases back I ended up turning off the whitelisting feature, so we've been treating it as a soft open beta for the time being. In fact, I was just at the Apple Store on Wednesday acquiring some new hardware to add to the build cluster.
The next release of App Inventor (this weekend if all goes well) includes some code level changes so that we can start migrating the iOS build code from our private repository to the public one over the coming weeks. If all goes well I am anticipating that the iOS builds will be turned on for production two feature releases after nb194.