'Join as party' will queue up your party (not raid, so max 5 toons) for the same AV. Since the random battleground feature introduction, queuing for AV is a pain. On my BG (reckoning horde EU) i have to wait between 50-70 minutes (at lvl 80). I've heard from other people that 60-69 and 70-79 is nearly dead.

On the other hand i can now queue up for AV whenever i want. Even at 3am i'll end up in a game since most people are using the random BG tool, they will eventually end up in AV. Whereas before the random dungeon feature, once after midnight there almost no AV's going on (except weekend and holliday).

This weekend is holliday AV on my BG, and queues are like 20 minutes now (specific queue for AV, with 5 man party).