Invites expire within 24 hours after the original email. You will need to go to manage accounts and under the pending button select the resend button and a new invite will be sent.
Pending Account Status
Until the new user finalizes the new account by creating a password, it will remain in pending status. Click the Pending button to see a listing of user accounts that remain in this status. To resend the new account email message to a pending user, click the small refresh icon for that user—as shown in the figure.