How do you access beta versions of Unity?

I was watching the videos of Unite Vancouver and heard something along the lines of “If you are a beta customer the good news it you can try this now”. Googling only turned up old data (Unity 4.0 public beta) and info about a Windows Phone Beta (which seems to be closed now), and searching Answers found only a question about 4.2 causing crashes with the askers particular setup.

How do you access the Unity beta? I found something like send Unity an email (regarding an old version, with no address given). Is it a very exclusive thing or something? What are the chances of being accepted? What sort of things seem to be taken into consideration?

(So many questions, but hopefully related enough for people to agree its best kept together!)

You have to be part of the beta testing group, which is by invitation. The 4.3 group is closed to additional members at this time. Occasionally they do open beta testing, such as for 4.0.