Trump had some of his club's members in attendance at the presser since it was at Bedminster. He invited them to watch him wrestle with the press, telling them prior to going on that "all the bad ones are here, watch what I have to deal with every day". So there were members there behind the press, watching and drinking. Some (not most, not all, some) did not have masks. Acosta was the reporter who complained about them, and Trump said they were peacefully protesting.
It was hilarious