Hey there. i saw the same thing, only after i got my renown up enough did the option appear for King Guy. He said i still needed to take the sword to his enemies first, but i was then able to pledge as a mercenary for one month to any lord of the kingdoms just like the main game and other mods. I'm guessing after enough renown you will be able to ( for other mods its like 150) but i'm not sure because i'm not there yet. You don't earn a lot of renown by enlisting in the army but you do get some good equipment. If he gets defeated in battle you lose renown as well, so to earn it you can go do quest and missions just like in other game modes.

I had to leave his service as a mercenary then manually rejoin as a mercenary for this option to come up for some reason.