I've been researching for a while how to do this type of request without using FQL (since FQL is obsolete). The only method I actually found working is to get the list of people who are committed in the /{event-id}/attending
event and then pick up the user's friends list and check this inside the server / application. I wanted to leave this processing for the Facebook server.
Does anyone have any ideas or have you ever done anything like this?