If your gym or coach invites you, you get an email with a link.
No account yet. The link confirms your address and lets you choose a password. Then the app opens with your gym in it: their colours, their logo, and what your coach has set up for you. The link works for one day; if it has expired, ask for a new one.
Already have an account. Then you’re not linked automatically. You see the invitation in the app on your home screen, and say yes there yourself. Until you do, nothing changes.
What your gym then sees. A coach at your business sees your workouts, measurements, nutrition and progress photos. By default only what you record from the moment you joined. If you want them to see your history from before that too, turn it on under Profile → What your business sees of you.
You keep your data. Everything you’d already recorded stays yours. If you later stop at this business, you simply take it with you.
A move from the Finder. If you got in touch with a business via I’m interested, it can suggest you move over. See Finding a gym or coach.