If you add a member to your party after accepting the quest, then there is a problem. That member can't enter the quest area. If you go back to the Hold and talk to the quest-giver priest then there is no option to add a new member. You can only turn in the quest: partially completed, successful, or unsuccessful. The second and third place the quest as a "completed" journal entry. The first is also of no help.

An option has been added to add people to your party in v881.