The role, or position, of The Witness–given form by Vince Fakhoury Horn–is an option that's always meant to be given by the social meditation facilitator. It is the option to participate as a silent observer, or witness, to the process, rather than participating out loud.
In Social Meditation practice, we want to make sure that the role of "The Witness" is clearly laid out and accessible to practitioners. There are two different ways that The Witness can be offered, depending on the order that participants are going in, during the social meditation (The When).
If the practices involves any kind of sequential order, then it's important to make sure that you ask participants, before starting the practice, if anyone would like to take the position of "The Witness"–i.e. a silent observer. Once someone commits to The Witness, they will be in that role for the duration of that round.
If one is facilitating a social meditation practice, where there is a spontaneous order–participants can meditate aloud when they feel moved–then the facilitator 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, as the facilitator, is to simply remind them that the Witness Role is always an option during spontaneous practice.
In some practice periods you may be be moving through multiple rounds of practice, some of which may be sequential and others spontaneous. If any of your rounds are sequential, make sure you invite people to volunteer as Witnesses before beginning the practice. If any of your rounds will be done spontaneously, simply remind them that they can always Witness during this time.