Soldato
Can you not separate your companions, give them their own parties and then combine them as armies when needed? I thought that was possible.
It's only possible after you've started a kingdom, and to start it, you need to own a castle.
You can apparently leave another kingdom, and choose to keep your properties. Or you can do what I did, which is to find a castle that's recently been won, and attack that one, since it'll only have 100 or so militia defending it, and you can take it fairly easily. Once done you can then find the kingdom you annoyed, and pay for peace, I think I paid about 50k. That gives you time to setup your own companions with armies, and your partner (my wife now heads up her own army, rather than governing). Once my kids come of age, they'll each get their own too.
I try and make sure my companions armies are A) Bigger than most bandits and B) Contain plenty of cavalry, so they can likely outrun most opposition.
Also something to note, your companions/lords etc don't seem to bother carrying much food, so if you are making an army for a siege, make sure you have enough food for everyone.