In Invasion gametypes, you are buddied up with another player, forming a sort of sub-team. If you are dead and your buddy is alive, you can spawn on him/her, and vice versa if you are alive and they are dead.
A red notification appears below your crosshair to tell you when your buddy is trying to spawn on you, and they won't be able to if you're fighting, but they can spawn stright into the side seats of vehicles.
If you are talking to your buddy, just ask them to spawn on you; especially if you need some backup, let them know.