Since you have been added, you can use all the beta firmwares that come along. The only thing is that you have to upgrade to the new betas whenever they come out. For example, if I'm on beta 1 and beta 2 comes out, I don't have much longer to use beta 1 before it expires and renders my device unusable until I upgrade to the next beta. This process applies for all of the betas except for the GM, which is basically the final version. Once you are on the GM, you don't have to upgrade until the next software version/set of betas come out. To downgrade, AFAIK you just put the device in DFU mode and restore to the current, non-beta version of the software. Another thing, betas are buggy. As far as betas go, though, these are pretty good.
Hope that made sense (it probably didn't, but oh well.) Sorry for the text wall LOL