Achievement says it all.
Dispatch missions can have up to 6 agents assigned for completion. The total rank of all agents assigned must be equal to (or greater) than the dispatch mission's complexity rating.
I recommend recruiting up to two of each agent class. If you do so, then when the dispatch missions become available, you will be able to send five rank 1 agents on the dispatch mission to the west - can't remember the name off the top of my head, but it only has a complexity of 5.
After you complete your next operation (minor or major), the dispatch mission will also complete, giving you any mission rewards and ranking up the assigned agents.