Having recently joined a gym after a few years of not going, I'm wondering whether things have changed.
Apparently, at my new gym, people can go into a spin class twenty minutes early to plant towels and bottles to 'bag' their spot.
If you go in just before/ on time, there may be a bike free. Most will be taken.
If you try to sit in the lobby, you'll be told "My friends are coming to sit here." and then told you're rude if you say you're "..just going to sit here for now, thanks." and asked whether "..you'd do that in a restaurant? Go and sit at a table where someone else was sitting?"
FFS