If you've been invited to a group
Open the link in the invitation email and sign up with the same address the invitation was sent to. That address is how we know the invitation is yours, so signing up with a different one won't let you accept it.
You'll be asked to confirm your email first. If you come back and find yourself somewhere unexpected rather than in the group — which can happen if you open the confirmation email on a different device, in a private window, or inside a mail app — don't sign up again. The invitation will be waiting on your dashboard with a Join button. It shows up wherever you land, in any of the four tools.
Invitations last 14 days. If yours has expired, ask whoever invited you to send another.