You can't refill your group if it was premade.

That said, I, a tank, with my relic and BC gear, always check the join party in progress box. Because in the event it does happen, it usually cuts the time of the dungeon a lot.

For example, I once joined an AK group that was on the last boss and the tank had ragequit for whatever reason. We one-shot the boss, and I got 80/40 tomes for not even 5 minutes of work and 1 minute of waiting in the queue.

Good deal IMO.