I'm sure if someone did a count there would be thousands in the States. We'd probably call them associations. As David T. mentioned, a lot are affiliated with State or regional golf associations that represent the USGA locally. We run a handful of events every year for various groups like tournaments for couples, employee groups, intercity club comps, etc. that are a bit different from corporate events because the associations are made up of volunteers like charity groups. If we are typical, multiply our 5 events or so events a year by the number of public and private courses in the US and you have a gaggle of groups. Add in the golf leagues, charity groups, corporate outings and it represents a large, important market for golf.
For example, I was at Pebble a couple of weeks ago and they were running a multi-day tournament for LA firefighters and EMT's that has been going on for forty years.
Obviously, filling up your golf course is good for business. Selling your services to groups creates opportunities for your food and beverage folks to operate efficiently, unlike the ebb and flow of daily tee sheet play which just doesn't put enough butts in seats to pay for the labor and overhead.