Have them check their spam folder for messages from you and email@example.com. If they don't see the invitations there, check if you've sent the invitations to that person's work email address.
Corporate and government email addresses are sometimes unable to receive our emails due to security firewalls. It's therefore usually best to invite your friends using personal email addresses.
If the invitations aren't in a spam folder and were sent to personal email addresses and there still seems to be a problem, contact firstname.lastname@example.org.