An invitation link is good for 14 days from the moment it was created, and an expired one cannot be revived. There is no way to reopen or extend it. Somebody with curator, admin or owner rights in the organization has to send a new one.
The clock runs from when the invitation was made, not from when the email reached you and not from when you opened it.
Do not forward your invitation link
An invitation is accepted by whoever is signed in when the link is opened. It is not checked against the address it was sent to. Forward the email, or open the link on a shared computer where somebody else is signed in, and that account joins the organization instead of yours. Treat the link as private, and check which account you are signed into before you accept.
What each message means
- "This invitation has expired." More than 14 days have passed. Ask for a new one.
- "Invitation not found or invalid." The key does not match a live invitation. Usually the link was broken across two lines by an email program, or the invitation was revoked, or a newer invitation replaced it.
- "Invalid invitation link." The address you opened carries no invitation key at all. Open the link from the email rather than retyping it.
- "You have been invited to join" with a Login to Accept button. You are signed out. Sign in, then accept.
- "You have joined" followed by the organization name. It worked, and you are being taken to the organization.
A new invitation cancels the old one
There is no resend. When somebody invites you again, the earlier invitation is deleted and a fresh link is created, so the old link stops working from that moment. Always open the newest email.
Your invites list only shows invitations sent to your account email
Organizations, Invites matches on the exact address on your account. An invitation sent to your university address will not appear there if you signed up with a personal one, however long you wait. Three ways out: sign in with the address it was sent to, open the link straight from that email, or ask for a new invitation addressed to the account you actually use.
What accepting gives you
You join with the role the invitation carried, which is Member unless a higher one was chosen for you. If you were already a member on a lower role, accepting raises you to the invited role. It never lowers a role you already have.
Joining also follows the organization. See Why you are following an organization you joined.