I have seen this at some national park lodges. They have small rooms but a nice lodge common area. The concessionaire turns the lodge common area into essentially a lunch room by serving drinks and "appetizers" in the common area. A huge volume of day bus tour visitors overwhelm the common areas ordering appetizer lunches and move around the furniture such as the coffee tables to turn them into dining tables. At another one, there is a nice but not large, pool, and the concessionaire figured out they could make extra money by selling day passes for the pool and pool showers to the people staying the the huge campground next door (which doesn't have showers) overwhelming the facility

The Hilton Diagonal in Barcelona sells access to their pool. They even warn hotel guests to make a reservation for the limited area available.

If I see a group in a hotel I am using (which I avoid like the plague but sometimes get caught out) then I always ask Reception what time is the group having breakfast, so I can avoid it.

Sadly the groups often have breakfast early and depending on the hotel and the group there may be nothing left, or nothing left that's worth eating, after they have gone.

With this sort of experience even only once I am likely to shorten my stay (even if the group has gone) and never, ever use that hotel again. If I want to be swamped and excluded by a group I'll join one, thank you.

Hotels have to learn and double dip like that at their peril. Unless they want to be left with the groups, that is.

I stayed recently at the MGM Grand in Las Vegas (same ownership as Mandalay Bay) and wondered why I saw several people bringing the inflatable tubes from the pool up to their rooms. Now I know.

I stayed at the Roosevelt Hotel a few years ago in Hollywood (classic hotel, and inspiration of Disney's Tower of Terror). The first night, there was a private function that had taken over not only the lobby bar, but the entire lobby. As I was not invited to the party, I couldn't even enter the hotel lobby, despite being a registered guest. Due to its location, there's plenty to do within walking distance, so it wasn't that big of a deal, but still.

I looked at two Hilton properties in Barcelona. One a convention hotel. The other has a pay to use pool. You can't win.

Aria casino hotel in Las Vegas has a separate "hotel within a hotel" called SkySuites.
They have a private pool SkySuites for guests only .They advertise that as one of the amenities.
A year ago they started selling seats at that pool for $100 ea.
Last summer I had to wait several times to get in as the place was full and a lot of seats were occupied by non guests.

How do you know they were not guests?

I love the site Kill Resort Fees, a Vegas page that continually highlights the ridiculous fees championed by Reno, and now Las Vegas, and catching on in other places. Being from Vegas, I'm old school and realize that the opening of the Mirage marks a point in time when the town I grew up in changed forever. But the resort fee, what a crock. Three years ago we had a reunion and stayed at the D downtown. At the time they charged a $29 per night resort fee for pretty much nothing. Even the wi-fi sucked. During our last reunion in January, we avoided resort fees by booking a vacation rental and others in our group booked hotels that do not stick you with this nuisance fee. I see some of the properties back-pedaling on charging for self-parking so maybe they are seeing a drop in revenues that equates to taking action. Until they are required to include resort fees in their advertising, the lure of what you think is an extremely inexpensive room rate will continue as a fraud on the consumer.

How did you check in if you weren't allowed into the lobby?