So I sent Yammer a question through their feedback collection tool:

Are we allowed to create collaborative networks of people that don't share email servers? I can imagine creating a network of people that yammer back and forth, for business reasons, but we're not all at the same company; we're working on a side project together. You guys interested in having groups like us on Yammer?

Maybe I'm asking that Yammer lose focus on the ONE THING it wants to do well, but maybe they'd be smart to let non email-affiliated people use their tool. 

I guess one problem with that, however, is that us non email-affiliated people probably aren't going to want to pay for access to our own little grassroots network.  Hmmm.