We're letting on some people at a time, quite few at first and more later on, to make sure we're not overrun with identical bug reports right away.
It's public in the sense that anyone can apply. However, we always ask that beta testers only discuss the beta on the dedicated beta forum, not here or in other places. This is important because the beta is still just that, a beta. There might still be bugs, issues and it's absolutely super slow while we use logging versions. It'd be pretty bad for everyone if someone posted, say, a comparison of our logging beta vs. a released version of Ableton Live 10 for example.
We'll start the beta with a logging version which, like I mentioned, is significantly slower than the real deal. The reason for this is that we've done many changes so even if this is a performance update, it's even more important for us to find out that we haven't broken anything—that's the kind of stuff the logging version catches. While in logging, you can still compare performance somewhat by turning some of the 10.3 improvements on/off via the preferences.
Further into the beta testing, there'll be a version without logging code available in parallel for pure performance testing.