I would look for other opportunities to volunteer and meet people...then check them out before you commit. Then I would call the head of your group or committee and say "Bob, I am so sorry, I have overextended myself...I am now volunteering with the food bank. Between it all I need to give up this group." Then you wouldn't be lying.
Personally, however, I'd just say some big lie like my mom was sick and I couldn't go anymore.