Process the event registration transaction in the NetCommunity plug-in in The Raiser's Edge. If they are still missing from the list after the transaction is processed, then review the following:

The event attendee list in NetCommunity will only display the participants that have opted in to be displayed on the list. If the attendee list opt-in attribute is missing from the participant's record, they will not be included on the attendee list in NetCommunity.

For more information on how to set up the attendee list, please refer to our article How to create a see who's coming list for event registration forms.

Per the article cited:
Note: Within the Directory Part, for Records to Include, you may leave it defaulted to All Constituents. The Records to Include does not change which records are displayed in the Attendee List. The Attendee List results are controlled by the Participant Attribute added to the participant records within the Event in The Raiser's Edge.