I would advise against using your companions as lords. They are valuable fighters in your army and do a lot more good there with their party skills and all. Besides, they tend to have low leadership attributes, unless you have build them with a lot of charisma, which means they'll never have an army of more than 60-70 since they don't level up after you make them lords.
Just make sure you have good relations with other lords, typically 30+ and they will join you eventually. As mentioned, keep fiefs open as they will attract lords to your court and build up a good army, both in your castle(s) and in your party so you have the military strength they are expecting from a king.