EDIT: also, it was rather hard trying to convince anyone to join my plots, what exactly do i need for them to join? a good relations with the character i'm trying to convince? or what?
Good relation with you, bad relation with his clan leader(if he is a vassal). You can use the master of ceremonies to improve relations and the master of guard to sow dissent on the same guy for greater effect. Also Having marriges and sending gifts help.