NRImate

Make your profile — name and city; LinkedIn, Instagram and a photo if you like.
Create private groups for the people you plan things with — invited by name; any of you posts.
Post an event; everyone says In or Out, and it is on once enough say In.

A public group is a business's: ask for one with its name, website and city, and NRImate checks it before it goes live.
Anyone finds it and joins with a tap; only the business posts, and everyone who joined says In or Out.
Or join one near you: + Group, then Join Public Groups — the map, by name or city.