Unless the AI is retarded, and won't get in. Here's the script:
(vehicle_load_magic [vehicle] [seats] [ai / biped])
The vehicle part is self explanatory. The seat is the seat that you want the ai to get into, such as W-driver for the driver of the warthog, and W-gunner for the gunner. Use "" if you want to load them all. For ai, either create a biped, and attach it to an encounter, and use that, or use the following:
(ai_actors [encounter]/[squad])
I'm sure you can figure it out. So if I had a vehicle named hoggy, and I wanted the driver seat filled with an encounter named driver, and a squad named squad, I would type:
(vehicle_load_magic hoggy "" (ai_actors driver/squad))