Send an error email when someone emails a discussion group they're not a member of.
When you email a meetup.com discussion list from the wrong email, your message doesn't get posted AND you don't get an email back saying you goofed up. Please make it say 'Sorry, your email isn't registered. You need to join this meetup (or meetup.com as applicable)'
I've wasted HOURS writing emails that didnt get through!!!