Hi Michael,
We understand you are unable to join Family Sharing because you're not seeing the invite on your device. We will do our best to assist.
In order see the invite, you must be signed in with the Apple ID the invite was sent to. Go to Settings > [your name] > Invitations.
If you're still not seeing the invite, have the the family member resend the invitation on their device by going to Settings > [name] > Family Sharing. Once there, have them select your name and then select Resend Invitation.
If you're already a part of another Family Sharing group, you will not be able to join a second one. For more information, click here: Set up Family Sharing
Best regards.