Interesting topic.
I am not sure how much growth in POG membership is part of the strategy and desired. Bigger is not always better and growth could, for example, change the dynamics of the group, require more administration, etc
However, I believe it would be quite easy to rapidly grow membership of the POG by implementing a couple of small changes on the POG website. For example, a highly visible promo slot on the main page with clear value prop, an easy way to join by integrating a simple checkout process, the ability to manage membership online, and especially the support of standard online payment methods as this would allow international members to join the group.
Personally I think the POG membership for $20 is the deal of the century and with some design tweaks and aggressive promotion [ad banners etc] the group size could probably be doubled in size within a year or two. Again, not sure this is desirable.
- Bernd