If you have some men with you and go to a lord and ask if he has any tasks he will say he promised to hire a band of mercenaries. I have tried this like 3 times, when im not related tp any faction :)

Oh, ok. I'll give that a try

Or you can fight under a lord directly as a soldier in his army which is a wonderful feature personally, definitely if you get promoted to Ji-Samurai. If you become friends well enough with that lord, and you wish to retire form his service you get to keep all the weapons/gear given to you while serving under him. If if you make it to max Samurai rank and retire with like 10-15+ friendship with said lord you get to keep that awesome top tier armor/weapons.