Open the link in your invitation email.
Sign in, or create your account, using the same email address the invitation was sent to.
Choose Accept and join. (Or Decline.)
You land on Home in your new tenancy. If you belong to other tenancies too, switch between them from the top bar. See Several tenancies.
Can't see your invitation? Make sure you signed in with the email address it was sent to, and that the address is verified.