If your attendees were not directly connected to your event (they signed in with a different email address, or are simply not on the list) or anyone on the app wants to access your event app, they can request access to your event from the app.
These requests can be approved from either the dashboard under Tools > Moderator Tools > App Access Requests, or from the Admin section of the app.
You can edit which of your admins can receive these emails under Event Content > Admin Settings. The admin(s) currently set to receive these emails will have a bell icon next to their name. Click on the pencil icon, and simply check/uncheck the box to your preference.