From Kings Island's Facebook page:

Gold & Platinum passholders can bring up to four (4) friends to the park for only $19.99 per person during Bring a Friend Days August 31, September 1,2 and 8. Your friends are welcome to join you for early rides on attractions like The Beast and Diamondback roller coasters. Simply present your season pass at the front gate admissions window to take advantage of this offer.

Expect those days to be more crowded than originally anticipated.

Cool, this may be of use to me.

Was planning on taking a family trip that weekend. Add this to the Dollar deals and it is apparent they are wanting the park as packed as possible. May be canceling that trip and waiting til Sundays in October.

They may want the park as packed as possible, but I don't think it's going to be super-crowded. The fact that it's $19.99 instead of $9.99 or free will help keep the crowds down.

If I had to guess, I would say that Saturday will be as crowded as an average June or July Wednesday, and Sunday and Labor Day will still be mostly dead. I just wouldn't expect walk-ons on everything that weekend. The 8th will likely also still be almost dead, but again don't expect walk-ons. But these are just wild guesses and could be way off.

Dynamic pricing.

And Fastlane passes will be through the roof...therefore....impossibly long lines. No thanks....

Can my guest get ERT with me on those days?

The answer is ...yes