The overall success of the dive is your concern. You must ensure that everyone else doesn't waste their money because you let someone with 50 jumps onto a 20-way. If a jumper needs a different jumpsuit, then help him find a loaner. Formations Plan the formations around the overall skill level of the people on the load. Dives become harder as the number of people increases.
Formations are easiest when everyone docks facing the center of the formation.
The more people facing out or sideways, the more difficult the dive will be. Be realistic in the number of formations planned. Most of the time on a skydive is spent doing transitions, not in a completed formation. Draw out the formations and pencil in the names of the people you want to do each slot. You can change this later, but have something ready before you call a dirt dive.