When you request to join a 👑 liege, you are asking permission to join someone's kingdom 🏰.
You may 🎈 propose 🏵 Companionship to any live player in your kingdom 🏰. They must accept the proposal 🎈. Upon accepting companionship, that vassal will no longer be in your recruit-able army and can no longer be placed in the defenses ⚪️🔵🔴 or the trapper 🕳.
Here link. Makes sense. Not difficult