Two ways to invite
An agreement is signed by two ClackID accounts. Invite the other person by email address, and only an account with that address can join. Or make a link and send it yourself, and the first other account to join with it becomes the other party. Either lasts fourteen days.
Sent to another address
If the invitation page says it was sent to another address, sign in with the address it was sent to. If that is not the address on your ClackID account, ask the person who invited you for a new invitation, as a link.
What signing does not do
Signing an agreement does not share your journal or switch anything on. Either of you can pause or end it at once; ending cannot be undone. Terms, on the Terms page, are the lighter way to agree a few things with your current keyholder.