An administrator needs to enable Session Collaboration in the VMware Horizon environment.

Collaboration is a feature that allows users to invite other users to join an existing Windows or Linux remote desktop session with both screen sharing and audio out features enabled. A remote desktop session that is shared in this way is called a collaborative session. The user that shares a session with another user is called the session owner, and the user that joins a shared session is called a session collaborator. A Horizon administrator must enable the Session Collaboration feature for the desktop pool or farm that contains the remote desktops that support collaboration.

One of the requirements to enable Session Collaboration is to use the VMware Blast display protocol for the remote desktops. VMware Blast is a protocol that provides high-performance, high-quality graphics and multimedia delivery over LAN or WAN networks. VMware Blast supports Session Collaboration by allowing multiple users to view and interact with the same remote desktop session simultaneously. Other display protocols, such as PCoIP or RDP, do not support Session Collaboration and will not allow users to share or join collaborative sessions.

Therefore, to enable Session Collaboration in the VMware Horizon environment, the administrator needs to use the BLAST protocol as a requirement. References: Configuring Session Collaboration, Sharing Remote Desktop Sessions, and [VMware Horizon 8.x Professional Course]