Hi Joe,
Welcome to Box Community and glad to help!
Please troubleshoot if your invitee's reporting that they are not receiving the invite notification. And have them follow these steps.
Note: Box "batches" email notifications before sending them out. This means an email may not be sent immediately after an action occurs. When multiple actions are batched/grouped into a single notification, the subject of the email will have the title of the top-level folder. This does not apply to Password Reset emails or Email Confirmation links.
Thanks for posting!