Hello there AByrner,
Thanks for participating in the Apple Support Communities!
Since you have confirmed that you sent the Family Sharing invites, let's have them check on their device for the invite as they don't see an email. The steps below outline how to locate this, and additional information can also be found here: Set up Family Sharing
Join a family group
Accept or decline an invitation to join a family directly from your device. You'll get an invitation by email or in a text message. Then you can respond right from the invitation. If you miss the email or text invitation, respond from your device Settings or System Preferences.
On your iPhone, iPad, or iPod touch
1. Go to Settings.
2. Tap your name.
3. Tap Invitations.
To accept an invitation, follow the onscreen instructions. When you join a family, you might be asked to confirm your account information and to opt-in to any features or services your family set up.
On your Mac
Choose Apple menu > System Preferences, then click Family Sharing. If you're using macOS Mojave or earlier, choose Apple menu > System Preferences > iCloud, then click Manage Family.
To accept an invitation, follow the onscreen instructions. When you join a family, you might be asked to confirm your account information and to opt-in to any features or services your family set up.
If you can't accept the invitation
If you can't accept the invitation, see if someone else joined a family with your Apple ID. Remember, you can only join one family at a time, and you can only switch to a different family group once per year.
We hope that information is able to help with clearing this up. If not, be sure they haven't switched between different family groups more than once per year.
Have a great day!