In Social Meditation practice, we want to make sure that the role of "The Witness" is always clearly laid out and accessible to practitioners.
If one is facilitating or hosting a social noting practice, where there is a sequential order to the noting, then it's important to make sure that you ask participants, before starting the practice, if anyone would like to be in the role of the witness. Once someone commits to that role, they will be in that role for the duration of the practice round.
If one is facilitating a social noting practice, where there is a spontaneous order (participants can note whenever they like) then you simply need to remind folks, before the start of the practice period, that they are free to move in and out of witnessing (and silence) as they like. You job here, is to simply remind them that the Witness Role is always an option.