Comments - Dafaq1029

Single Player

You need enough renown to get the option. Once you do it will say something like "My liege, I want to fight by your side!" or something like that. More than likely he will say "Bring the battle to my enemy, then we shall talk." But all you have to do is improve your relation with him by doing missions and task for him. Then eventually he will let you join by saying a oath and stuff.

Btw, don't talk to a vassal of the kingdom you want to join, talk to the King, or you wont get the option.

Hope this helped!