If that doesn’t work, clickCANCEL INVITEand re-invite the member by clickingSEND INVITE, and re-entering their details.
Tip: If you're using the Premium Duo plan as a manager you can perform these steps by visiting your Duo page.
Secondly, make sure that your family members have checked the Junk/Spam folder in case the invite was redirected there by mistake.
If that doesn't do the trick, try sending the invites to alternate email addresses. The email address doesn't need to be associated with any Spotify account as long as they log in using their personal Spotify account's log in details.
Once each family member has received their invite, you can ask them to follow the steps here to join your plan.