Support REST API to Fetch Active User Count for a Group

This feature request aims to introduce a dedicated REST API endpoint that allows developers to retrieve the current active user count for a specific group in real time. The API should return the number of users who are currently online or actively engaged within the group, based on presence or activity status. This functionality is important for use cases such as displaying live participation metrics, enhancing user engagement insights, and enabling dynamic UI elements (e.g., “X users active now”). The endpoint should be scalable, performant for large groups, and consistent with existing authentication and group management APIs. Additionally, considerations around how “active” is defined (e.g., online presence, recent activity threshold) should be clearly documented and configurable if possible.