I've already gotten responses on twitter and facebook from a majority of people that show up at the casual meetings (get on the twitter you crazy luddite). Spamming all the members may be overkill.
With all due respect, I wouldn't consider a meeting notice as spam. Twitter and Facebook were not established as primary communication channels for the group. I believe we did do a notice in the past and at least a couple people were very appreciative and said that otherwise they would've shown up at Common Roots and missed the meeting.