It depends. My native language tracker forexample opens up for signups at Christmas time every year, making it easy to join. Outside of that I believe you can just make an application and depending on how good it is, you'll be accepted.

But this is different from tracker to tracker, so I can't say how it is exactly for your language. But what is pretty much standard for all private trackers is that you can get an invite if you know someone already there.