Hi @Tom Slayton
Could the SMS user receive the invitation when adding to a new group?
To exclude the client issue, we firstly recommend you try to use Teams web client to see if it works. If there is no issue in Teams web client, it recommends you sign out your Teams client and clear Teams client cache file by following these steps:
1.While holding the Windows key, press R to open Windows Run, then copy and paste the following into the box and click OK:
%appdata%\Microsoft
2.Find the Teams folder, then right click it and select Delete
3.Relaunch Microsoft Teams.
If the response is helpful, please click "Accept Answer" and upvote it.
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.