Alliance missions can either be missions shared by your alliance members or spawned by your alliance admins.
Since everyone gets the full reward for a mission, independent from what they contributed to the mission.
Example: The reward for accomplishing a mission is 20.000 Credits. Max sent 200 vehicles to that alliance mission. Bob sent 1 vehicle to that alliance mission. Once the mission is accomplished, both get 20.000 Credits.

It is good practice to share big missions with everyone, so everyone has the possibility of reaping the full rewards of a mission.