A unit can get inside a vehicle by moving to within 2" of it. The whole unit must be within 2" of the vehicle before it gets inside. If some models are out of range their compatriots will wait until they can mount up together. If the vehicle moved before its passengers got aboard then it may not move any further in the same turn. If the vehicle has not moved beforehand then it can move up to 12" as soon as the unit is on board.
An infantry unit can embark into a vehicle in their assault phase if it consolidates and moves to within 2" of the vehicle.
Diagram: Embarking Before Vehicle Moves. As the vehicle did not move before the troops embarked, it can move up to 12" that turn.
Diagram: Embarking After Vehicle Moves. The vehicle moved before the troops embarked, so it cannot move any further that turn.