I'm visualizing a grid of checkboxen, and the axes of it may be flopped, so one could either easily do the "check all powers for this one person" or do the "give all people these powers".

Powers come pre-populated. (Bundles? hmm.) Grid would be silly if all members of comm were on it; there would be a thing to add them (checkboxen in membership roster + text-entry on manage page?) and then grid happy funtimes ensues. On checking, square on the grid holding checkbox highlights, so one can see more easily if there are any breaks (you have no-one holding one particular thing, or a person lacking a specific power).