Yes, anyone can try a beta for which they’ve signed up for. Then you just select one of the beta channels for which you’ve registered under the automatic update settings.

The Public Beta is free. I tend to try stuff out around PB4 to get ahead of the curve so I can teach my parents any new features.

Developer Beta is in the process of weeding anyone out who isn’t actually in the developer program.

Customer Beta is invite only by whatever criteria Apple uses. Seems to be (ironically) employees, power users and people with a long history of engaging with Feedback.