You can sign up at https://rokutestingportal.centercode.com. I don't know if they're currently accepting applications or not, but since we're nearing the complete rollout of 9.0, it shouldn't be too much longer before they start going through the applications again and adding more (we always need more). Be sure to fill out any and ALL applications on centercode, and put in as much information on your local environment as possible (TV make/model, receiver make/model, et al).
I can also put in a word or two to the Beta Coordinators to get you on the Beta Testing list (no guarantees though). They want active participants willing to dedicate time each day/week to providing feedback and bug-discovery. If you're only in it for new or preview builds, however, I don't advise it. NDA's can seriously impact how you post on public forums (such as here). atc98092 just said about as much as he's able to say due to NDA, for example
C. Shawn Smith
The Cosmos is all that is, and all that was, and ever will be. -- Carl Sagan