How do I convince a lord to join my faction? (am I missing something...?)

When I try to send an emissary it only gives me the option to send a message to the rulers either a peace or vassal message. I have captured a lords castle and to try and get friendly with another faction I plan on giving them a castle. Is there a different way to convince someone?

Hello, check you companion persuasion skill and charisma if I remember correctly and try to weaken the faction that you want to join you.

Sorry after checking, I confused with another mod, you can't convince another faction to join you.