Boston Classical Meetup was created in August 2011 to provide a general classical meetup group for veterans and rookies alike. We attend all types of classical performances: symphonies, operas, chamber works, etc. in all kinds of venues - even house parties! (think Schubertiade...) The Boston Opera Meetup (http://www.meetup.com/BostonOpera/) works in tandem with the Boston Classical Meetup for opera events.
We will usually be meeting after performances, sometimes before, and occasionally during. Pre- and post-performance activities (lectures, receptions, parties, etc) are usually part of the meetup itself, so be sure to check the details!
Check out The Boston Musical Intelligencer calendar page (http://classical-scene.com/calendar) and feel free to suggest a meetup. Please note that you would likely be responsible for coordinating the event.