You can ALWAYS accept people for your own battles. The rank gives you the ability to accept people for OTHER peoples battles.
Example:
Faction with leader A and B, and Grunts A, B, C and D. Leaders Rank 10, Grunts Rank 1.
Grunt A gets attacked. He can accept people to his battle and so can Leader A and B. The other Grunts can't do anything with it (apart from applying for the battle ofc).
Leader B attacks a village. He can accept people for the battle. So can leader A. None of the Grunts can do anything.