The Role of "The Witness" is critical in terms of how we teach & practice Social Meditation. It means everyone has the choice to silently witness the practice, instead of participating out loud.
The easiest way for folks who are connected virtually to identify themselves as a Witness is to invite them to turn off their video feeds if they wish to Witness. This way they can choose to enter and the exit the Witness role as they like, with a clear visual cue for all those participating.
When you introduce the role of The Witness in-person it's important to have either A) a dedicated place where witnesses can go sit during the session or to have them visually identify themselves to their group–particularly in the case of practices following a sequential order–so that everyone in the group knows who is witnessing.