Hey ScottLowenbrau,
Is this the only link Safari has trouble opening? If you're unable to resolve the link invitation, you should be able to send the invitation a few other ways as referenced in the "Add a family member with an Apple ID" section from Add a member to a Family Sharing group on iPad. It'll be listed below.
"The organizer of the Family Sharing group can add a family member who has their own Apple ID.
- Go to Settings
> [your name] > Family Sharing, then tap
in the upper-right corner. - Tap Invite Others, then follow the onscreen instructions.
You can send the invitation using AirDrop, Messages, or Mail. If you’re near the family member, you can also tap Invite in Person and ask the family member to enter their Apple ID and password on your device."
Give one of those other methods a try, and if issues remain, be sure to post back .
Take care.