If they don't give you the option for the dialogue it means they want way too much $$$ and you can't get them. I'm going to paste in some good info in minute brb. (couldn't find it but I just wrote all this recently)
So to recruit a clan
1 they need to be wanting for something, so look for homeless clans with no fief, if they have a fief it will be much more expensive and often not possible to recruit them. The worse off a clan is financially the better.
2 The more powerful you are (parties and garrisons) the more attractive you are to vassals. This mean after you have a few vassals it will eb easier to get more.
3 Your relations with the clan and thier relation with thier king does matter, but no so much that it will over come the 1 and 2 issues. Power and them being homeless and poor is most important.
4 Homeless clans will want 250K or so to join you. The price can go down when you're more powerful or they like you, but count on paying 250K for your first few clans to join you.
5 Once you have 1 use the "return fief" (might be called differently)option to put a castle up for election and give it to them, or siege and take a castle for them to secure them like a good king.
6 side note. Get some merc clans when you can find thier leaders, don't be too picky, just any merc clans early on will increase you field power right away, I think it increases you power value (to potential vassals) but I'm not sure .