I thought I'd seen somewhere that the tribespeople are humans, but I can't find it in the threads. Maybe I imagined that.
Invisibility can make others invisible. It's one target per second-level spell slot and we've got 5 of those between us,
so we could make the whole party invisible, but it would cost almost all of our spellcasting resources (though we could get 3 slots back on a short rest).
Edit: oh yeah, concentration. Blerg.