I am beginning to think a little differently about volunteers as this discussion develops. I notice that I have been thinking of volunteers as specific people, but I suspect I need to change my thinking around.
If I start with clearly-defined accountabilities that capture what it is that the organisation wants done, then I can more easily distinguish those sets of accountabilities which will be the responsibility of paid staff in the relevant roles, and those things which can be done by unpaid staff. This offers clarity (to ensure that people can choose to contribute but are not in effect 'working' for nothing), by putting responsibility on the organisation for securing or resourcing what is functionally necessary, rather than getting unpaid staff to fill in any gaps.
This makes it easier for me to see volunteering as a role which may or may not be assigned to a given individual. The conditions as well as expectations are clear, and there do need to be specifiable benefits to the person choosing to accept the volunteer role. The volunteer role will be what we choose to make it, including having a purpose which meets the needs of the organisation, or aspect of relevant service delivery, as well as the needs of the person, who wants to be of service and make their contribution.
In other words, the previously tacit understandings associated with volunteering become transparent parts of a formal agreement between the organisation which offers volunteering roles, and the person who offers to accept the role, with all that it clearly entails.
This is pretty similar to the more clearly contractual arrangement outlined by Brian, which is an arms-length, time-limited association with and conformity to an Holacracy-based organisation of a much tighter legal and financial kind.
I am imagining a sub-circle dealing with volunteering support for the organisation's overall purpose, to give enough space for the specification of differing kinds of accountabilities and therefore roles, and the possibility of evolution of this particular relationship between the organisation and the outside world, including appropriate representative links.