[Steam] notify server and client when a player joins or leaves a lobby session

Hi,

Our game utilizes Steam lobby sessions by creating presence sessions. Creating and joining the lobby as well as travelling to the actual game from the lobby works well.

However, it would be great if all lobby members were notified when a player joins or leaves the lobby, before the match starts. If it was possible - in addition to the former - to query all lobby members to display them on the UI.

I have been digging through the code but could not find any way to react to players joining or leaving the lobby before there is an actual game running. When an actual game is running, the game mode login/logout functions may be used, but those are not an option before the game starts.

Has anyone done this before or is it simply not possible with UE 4.12.5?