The long answer...
Users receive an invitation email if an administrator manually adds them to their venue's list of users (assuming the relevant notifications are enabled).
The only real purpose of the "invitation email" is to help the user choose a password (or social connector) to login.
As such, if you find yourself asking whether it's possible to resend the invitation email, consider just requesting a password-reset email on the user's behalf. The email they'll get will serve the same purpose: help to choose a password and login.