I've never heard of a Triathlon group here in Boston so if you have the time and the energy go ahead and start one.
I'm in the Frontrunner group. Some of them have done Triathlons. I'd be happy to post a message on their mailing list if you send me one. (Or you can join the group yourself.)http://www.frontrunnersboston.org/
The only cycling group is the OutRiders which just had their century ride the other weekend. But you could post a message to their Facebook page.http://www.outriders.org/
The Chiltern group has some riding events too:http://www.chiltern.org/
Lastly you can contact LANES to see if any of the swimmers are interested:http://www.swim-lanes.org/stage1/php/welcome.php
It was my goal to go my first Triathlon this Fall but I couldn't get my swimming up to speed over the winter/spring. I'm focusing on running and cycling. One day...