When I grill steaks, I do potatoes like this:
Slice potatoes into coin shaped disks (like you see in scalloped potatoes). Do the same with an onion, then pull the onion disks will pull apart into rings.
Pull out maybe a two foot length of aluminum foil (better if heavy duty stuff).
Put a little olive oil on foil and smear around (so things won't stick).
Layer potatoes, onions, potatoes, onions, potatoes. Sprinkle olive oil over each layer as you build them up. Add salt and pepper to each layer too. Don't fret about putting too many onions in. Load it up.
Close up foil, rolling edges to prevent leaking. Add a second layer of foil if you are using the lightweight variety.
Grill foil packet alongside steaks (start before steaks because potatoes will take longer). Turn occasionally, carefully so as not to tear the foil.
Total grilling time will be 30 minutes to 45 minutes in my experience - depending on how fat you stuffed the thing. The outside layers will brown and that is the best part IMHO. If you like that browning too, just do three layers - potato, onion, potato - and grill for a shorter time. It's usually easier to make multiple packets rather than one huge packet if you're feeding a lot of people.