If you receive an email titled "[Name of someone you know] Has Invited You to Join Crisis Cleanup," it will contain an activation link. That link expires after 7 days. If you click that link after it has expired, you will receive an error.
If this happens, simply ask the person to send you a new invitation by visiting the dashboard and entering your email address under the "Invite Teammates" section and clicking "Send Invites." You will receive a fresh invitation. There are no limits to the number of invitations you may receive.