If a user's account invitation email expired before they could create a password and login, administrators may resend it from Settings > Users or Settings > Network Users.
Search for and select the user who needs their invitation resent.
Select the dropdown menu on the far right of that user line.
Select Resend Invitation.
Select OK to confirm and a new invitation will be emailed to them. Invitations are good for 24 hours.
Example of resending a user invitation.