Looking at this ability again since maybe I'll live long enough to get it someday.

So, I'm 10th level, I have the ability, and I have some castle to storm or something. I need 20 people to come help me.

Does this ability actually summon these people, or do I give a speech at a fighter's guild, convince my fellow prisoners to stage a jailbreak, or put up posters and wait a week for them to arrive, then the 1 day starts once they all show up?

Or is it just mean to let me give extra HP to my existing companions and men-at-arms?

It doesn't say anywhere that these allies just appear, and in any case doesn't specify who would show up if they did (just that they are the barbarian's level or less).

It's nice to give your friends an extra full hit die, but is that all this does?

I think in function, it just gives you a way to let your equal/lower level buddies some extra hit points.

How that plays out in game would be up to the CK and what kind of game they are running.

R-

The inspiration is likely from 1st edition, where the horde is called for support, sort of like summoning but not quite.

It can be interpreted numerous ways.

I personally find it quite powerful.

In the CKG under Land as Treasure: Barbarian, there is information on "Calling". It is described as "his ability to enter his local tribal group and raise bands of warriors". It's also described as "his ability to encourage men to like minded deeds". The barbarian can "call" other barbarians starting at level 3.

I would rule that this ability can only be used on barbarians of their tribe, or at least those of similar outlook. At tenth level they would receive the bonus HP.