Q u o t e:
Yeah, I have loads of questions about rated BGs. It sounds like it's "Guild BGs With Ratings", rather than Rated BGs; if I'm not misinformed, all members of a BG "team" have to be in the same guild, no?
Your guild membership has no impact on rated BG teams. You can form a raid to join up no matter what guild you are in. However, if you join a rated BG with the minimum number of required guild members, you will earn guild xp for a win. You will be notified in the top right of your screen via a guild banner that you are in a guild group.
It actually sounds like there is a lot of confusion going on with players about rated battlegrounds in general. I will get a basic FAQ together and get it posted ASAP.