When you invite another player to party, they appear offline until you're close enough to see them.
Edit:
This also occours later in the game. For instance:
You join arena. Upon leaving arena, your partner once again appears offline, until you run to him. You're unable to queue up until you're in range of your partner again.
Upon leaving arena, group bugs out for one or the other in the group so you can't see the other one. Which means, you can't queue for arena again and has to relog.