ai_follow_distance <ai> <real>

maybe ai_free <ai>

(ai_vehicle_enterable_distance <unit> <real>)
sets a vehicle as being impulsively enterable for actors within a certain distance


(ai_vehicle_enterable_team <unit> <team>)
sets a vehicle as being impulsively enterable for actors on a certain team


(ai_vehicle_enterable_actor_type <unit> <actor_type>)
sets a vehicle as being impulsively enterable for actors of a certain type (grunt, elite, marine etc)


(ai_vehicle_enterable_actors <unit> <ai>)
sets a vehicle as being impulsively enterable for a certain encounter/squad of actors

There is also command lists, directly loading them in vehicle_load_magic, unit_enter_vehicle, etc. Plenty of ways to do it, but im assuming this is what you want, where they can enter any vehicles they feel like that are nearby, right?

Apart from the solutions posted above, you also need to check if the AI bipeds have the necessary animations to enter the vehicles. EG. W-driver can be entered by Spartans but not by the default Halo 1 marines (basically whatever that the seat label for each particular seat is named).