Please note that the email sent from the app might take up to 10 minutes to arrive.
Make sure you entered the email address correctly. To verify, go to your contact list (in the app) where you can see who has access to which gate.
You can send a simple message to your guest's email address just to test whether the email is correct.
Some messages might land in Spam folders or are caught by Spam blockers. In case the recipient doesn't receive the invitation, it's recommended to check the Spam folder (in Gmail account, it could be in “Promotion”).
Another solution could be checking your internet connection on your phone.