Marshals who have the king's authority to gather armies. If you're not the marshal, you can only assemble armies out of your own clan armies or friendly lords.
Instead of costing influence to summon lords to your army, it should just be a yes/no decision depending on how much they like you, your reputation, their personality, whether you're the marshal or not, whether they have enough troops, whether they have enough food, how long they've been in the field and if they need to rest (in warband lords had to rest in cities/castles after being in the field for too long), etc.
And of course if you're the king you automatically have marshal powers too