CoP Nucleation

There are two ways of forming a CoP (I say two cause I don't believe in the third, the "built" one):

1) "spontaneously":
I don't believe it is really spontaneous, it's more like somebody is doing the work without knowing what s/he is doing... according to the Johari window, it's the state of "unconscious competence". It's is more or like the case in which an "unconscious" nucleator starts aggregating a CoP not knowing what s/he is doing:
2) "nucleated":
you follow me on this, my background is in medicine. Stones get formed in a liquid that is supersaturated with a given solute. However, supersaturation is necessary but not sufficient for a stone to get formed. It takes a little "piece of something", like a cluster of some 5-10 cells, a kidney cylinder, few bacteria. That "piece of something" in a supersaturated solution starts what is known as stone "nucleation". So, what I do in my practice is to search for the supersaturated solution (aka: a 10-20 member network among 3 or 4 departments, like Verna Allee would say) and then I search for the "piece of something" to shove into it. The piece can be my "formal/informal" leadership, but most often it is my detection of a subgroup of people that could be autochthones leaders, a topic that can create debate (and therefore emergence of a leader), sometimes shifting focus/technology attracts the "right" leader. It depends.

What you have to search for, however, is a particular kind of leader, a servant-leader (if you are familiar with Greenleaf.. if not, see here).

On difficulties in recruiting:

That might also mean a good thing, such as you nucleated a CoP with a peculiar identity, that isn't a fit for everybody under the sun. It might also mean the CoP is functional to the point that members are having productive lives without having to always resort to the CoP. However you are right, there is a danger in that and Gongla and Rizzuto in their "Where Did That Community Go? - Communities of Practice that "Disappear" described pretty well.

Volunteer recruitment (and CoP members are *volunteers*, NOT workers!) works in a different way than workforce, so one reason might be it. Have you tried administering the VFI to your CoP members and see what their motivations are? Basing on that, you might want to craft an announcement attracting volunteers with similar motivations. Dig in the work of Clary, starting from
this).

This paper is the one that explains the connection between motivation and kind of advertisement: Clary, E. G., M. Snyder, R. Ridge, P. Miene, & J. Haugen. (1994). Matching messages to motives in persuasion: A functional approach to promoting volunteerism. Journal of Applied Social Psychology 24, 1129-1149.