Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
In response to the following error I have figured out the solution:
"Looks like this invitation link is not working
You’ll need to contact the person who invited you and ask them to share the subscription with you"
Microsoft does not know the answer and neither does their Bing AI since this is an unresolved bug.
The problem will occur if the person you are attempting to share the subscription with is already a member of your family safety group.
While logged in to the account that is attempting to send the share go to https://account.microsoft.com/family/home
You will likely see the person in your list of family twice. One of them will be pending invitation.
Remove the person from your family group (not the one pending invitation). Once they are kicked out go ahead and accept the invitation for them using their account.
This should now work and they will be back in your family group. You will likely have to change any family sharing settings in Windows after all this and you may need to sign out of MS desktop apps and back in (for the added user).