If your at&t or Verizon you'd have to sideload beta as they don't participate in betas, enrollment most likey full as is been a while. As of official release they usually depend on the beta issues being reported and resolved before they release to the public hence the beta tests. Rumors claim before the end of December
Anyway, you need to look for a Beta banner on the main screen of the Members app.
If you don't see it, the beta is closed ... or you are not able to participate.
Some carriers, like Verizon, don't allow beta installs on their locked phone ... however, if you have an Unlocked phone on Verizon, you will be able to participate.
Other carriers have similar policies for their locked phones.
