Nigel, you are helping direct people to the Travel Group Meetings Forum by posting this question!
In my experience, someone notices another poster's location and if it's nearby, suggests meeting somewhere. Panera seems to be popular. The groups seem to grow from there.
As for meeting in Europe, I've noticed some invitations popping up occasionally, as I'm sure you have, along the lines of "I'll be in Florence June 12-16, anyone want to meet for coffee?"
As for finding the Meetings Forum, how does anyone find a particular forum (like packing, travel partners, etc.)? Read the Webmaster notice or check the directory.
In reading this very pedestrian answer, I hope you weren't looking for something more existential.