Was your group entirely premade (shout group)? Right now there's no way to refill a group that was entirely premade. Otherwise yes you would have gotten a popup asking if you want to replenish. If anyone is on in your FC you could ask them. Odds are if they check the join in progress box they'll land in your party. Our FC leader was able to jump in and help a couple of members who had their tank drop out at Isgebind in Stone Vigil by doing that.