Ask to group with them. Whatever instance they're in if it's not the same as yours, you'll be asked if you want to move to the instance they're in - unless that simply doesn't work in pvp (and I don't know, I don't pvp). But that works in pve....
I know about that already - I was thinking when pulling up your guild roster or friend list when it says Cyrodiil it would be nice if it said Cyrodiil (Ravenwatch)