This is the commonest thing that goes wrong with an invitation, and it is the least serious. Somebody meant to accept, life happened, and the window closed.
The fix
Go to Family & Friends → My Circle → Our People.
Tap Invite someone to strengthen the Circle again.
Enter the same email, and send.
The new invitation replaces the old one. There is nothing to undo first.
If they see "Access Denied"
That is the same thing wearing a blunter face: the link they opened is expired or has already been used. Ask them to open the most recent invitation you sent. If that still will not open, send a fresh one.
If it keeps expiring
Try the private link instead of the email. Some invitations land in a spam folder and are never seen, and a link you send yourself, in a text they are expecting, tends to get opened. See What is the difference between an email invitation and a private link?