Doug,
The ground rules for IETF working groups suggest that the mailing list
is closer to the right unit. It's legal to run a working group
without having any face to face meetings. Of course, face to face
meetings are useful, and othings often happen at the working group
meetings that might take much longer (or be different) if there
weren't any meetings.
Conversely, the mailing list is essential. It's not legal to run a
working group without a mailing list. Although meetings are useful to
exploring ideas and striving toward consensus, in a strict sense it's
not possible to reach a decision without involving the mailing list.
The PEM WG mailing list is this mailing list, viz
pem-dev(_at_)tis(_dot_)com(_dot_) I
don't known its size, but it's pretty big. Also, many addresses are
exploders, so even if we measured the size of the top level of the
mailing list, it wouldn't tell us how many people actually get mail.
Steve