If you are familiar with jQuery and follow their news, you would have heard that they switched from Google Groups to Zoho to host their forums / discussions. John Resig gives a number of points on the failures of Google Groups in his blog post and I can see why.
Since I joined several groups that I found of interest at some point, I had to some Google Groups cleanup. I removed several of the projects that I was no longer interested in and found a new preference “Do not allow group managers to directly add me to their groups” which was defaulted to being unchecked. I am not certain of this but my understanding is that it allows a group manager of any group I am a member of, to create a new group and add me to it.
It almost seems spamalicious to add that setting and defaulting it to “Yes, please allow Group managers to invite me to new groups that can spam me and pass my information to them”. When I cleaned up my groups, I thought I found a few groups unfamiliar and I am still not sure if it was just Group Managers who added me or if I accidentally joined the discussion forums at some point.
Check your settings and leave feedback on whether you also have the same settings.