If you can’t get in, go through these points. They’re in order of how often they turn out to be the cause.
The right email address. You sign in with the address you signed up with or got your invitation at. Search your email for that invitation; the address it arrived at is the right one.
Signed up with Apple or Google? Then choose Continue with Apple or Continue with Google again, not the password field. An Apple or Google account doesn’t have a separate password with us.
No password chosen yet. An invitation contains a link to choose a password. Without that step your account does exist, but you don’t have a password yet.
The link has expired. A confirmation link from the app works for an hour, an invitation from your gym for a day. Request a new one, or use Forgot your password?.
Forgotten password. There’s a link on the sign-in screen. Enter your email address first; you’ll then get an email. Check your spam folder too.
Someone else was already signed in. If you open a link from your email while someone else is signed in on that phone, it won’t work. Sign out first and open the link again.
Still not working? Ask support, or contact your gym. Mention the email address you’re using.