While in Arbitor form (Healer role) Boon of souls does not shield the 3 other players. It shields the caster as a personal shield and no one else. Easily reproducible, just need 2 people and for that second person to take damage. Now i do admit, i took 6 years away from the game. Just right before Home Turf, and came back right after Titans content, so there might be info i am missing. I do know that arbitor has the option of both dps and heal powers, but i would suspect that due to me being in healer role, Boon of souls should shield up to 3 additional people even if in arbitor form.