When you request to join a π liege, you are asking permission to join someone's kingdom π°.
You may π propose π΅ Companionship to any live player in your kingdom π°. They must accept the proposal π. Upon accepting companionship, that vassal will no longer be in your recruit-able army and can no longer be placed in the defenses βͺοΈπ΅π΄ or the trapper π³.
Here link. Makes sense. Not difficult