Does it have an affiliate program?

Yes, it has. You can read about the affiliate program here

I don't understand clearly how it works. Can you explain to me?

You can earn more if you add all supported pools to your minerboard account and invite as many miners as possible

So what if I use only one pool?

Then you can receive additional income only from it.

So how much more income can I get with this affiliate program?

It all depends on the number of added pools and invited users.

Is there a static percentage?

Yes, at the moment there is 30%

How is this percentage calculated?

It depends on the commission that the pool already charges you.

Do pools make money on referrals I invite? Are they therefore sharing a commission?