You'll first want to go to your Invite & More tab of your conference page and make sure they were sent and delivered. You can check that in the Invited List:
If nothing is there, make sure you know how to invite people. Please visit the section of the Help Center that covers this, "How Do I Invite People to My Webinar?"
Once you've invited people, on the Invite List you'll be able to see who invited and who opened, clicked and registered from your emails:
If you see a person's email here, and yet they haven't received it, they may need to check their spam or junk folder. If they can't find it there, try sending them another invitation. If they still don't get an invite, please email firstname.lastname@example.org