Get your gear first, then get the pack that fits it.

Suggestions: Six Moon Designs, ULA-Equipment, Gossamer Gear, Granite Gear. I suspect that if you are carrying gear for others, you will not want a completely frameless pack such as those made by Mountain Laurel Designs, Z-Packs, Gossamer Gear, etc. In other words, you'll need more capacity and some kind of frame or backstays that transfer most of the weight to the hip belt.