Coordinate first with the event organizers. Photography consent is often covered in the event terms, so confirm what attendees agreed to and ask the organizers to help communicate the policy.

For people who do not want to appear in photos, use an easy visual identifier such as a clearly marked or color-coded name badge/sticker, if the organizers and attendees agree. That gives you a practical way to avoid photographing them when possible and to identify them later if editing is needed.

It can also help to keep a reference record of those attendees, such as a name list or identification photo, so you can review images afterward and remove or blur them when necessary.

A simple written release/notice explaining how images will be used and how opt-outs will be handled is also a good idea.

In short: confirm the policy with organizers, make opt-outs identifiable during the event, and maintain a clear post-processing workflow for removing or obscuring those people if they appear in group shots.