IF your employee is not receiving the invitation email, then please follow the below steps for troubleshooting and other alternative solution.
Check the Web Application
1. Check that all emails sent from the system goes first to the Email Que. Check that there are no pending emails residing inside the email ques by going to Notifications > Email Que
Wait for the email to be processed and cleared.
2. Check if the system has already sent the email by going to Notifications > Email Log and search for the email by clicking on the search icon
3. if the employee has a Gmail email account, then click here for troubleshoting Gmail issue
4. if the employee email account is not a Gmail account, then let the employee check his junk folder and let hi also check the "All Mail" Folder