Diagram: Embarking Before Vehicle Moves. Troops converge on and board a stationary vehicle before it moves. Caption: "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 moves first (arrow), then troops embark. Caption: "The vehicle moved before the troops embarked so it cannot move any further that turn."
Models may disembark from an open-topped transport vehicle and re-embark as part of a consolidation move. Apart from this, no models may embark and disembark in the same turn.
Extra Note For Clarity: An Independent Character may join/leave a unit during its embark/disembark move, just make it clear to your opponent what is happening.