Pariah have two main roles in group combat: Support, or Damage.
First, for Support, Pariah has the ability to give everyone a passive cure with the leech logograph in Memorium. In addition, the Wave ability in Charnel provides the ability to push people out of room, which can be used to separate enemy groups.
Damage wise, sting/wrack is your main option. It is quite good but has wind up, since you need to trace twice to use it.
Pariahs can also single targets out with Wave and 1v1 them, potentially bursting them down quickly or just removing them from the fight as well. In this sense they can kind of act like an assassin character from League of Legends. This is especially true when you utilize one of the tablets available, but I won't comment on that much further.