Is it illegal to use photos in a photography group? How to report it?

Illegal. Using the photos in the photography group without others' consent infringes the copyright. If the other party uses your unauthorized photos, please actively negotiate with the other party and ask the other party to delete them. If you refuse to delete it, you can bring a lawsuit to the people's court and ask the infringer to stop the infringement, eliminate the influence, apologize, etc. If it causes you economic or other losses, you can claim compensation.