Your studio added you and sent an email. One button opens your account. There is no password to set.
The subject is "<Your first name>, your <studio> account is ready". Press "Open my account".
You see "Welcome" with your name already filled in. Correct it if it is wrong, then press "Looks right". Pressing it also accepts the Terms and Privacy Policy linked under the button.
Anything your studio already did is there: classes they booked for you under My Bookings, and your passes and credits. If the studio has a waiver, you are asked to sign it first.
The button signs you in with one tap for about an hour after the email was sent. Later than that, it emails you a code of 8 digits and asks for it. Open the new email, type the code, and carry on. The button never stops working.