J., that's pretty rough. It may be too late for the Labor Day party, but perhaps you can start getting invited again if you take a passive route.
Perhaps just say to them, "You know, we haven't been able to spend as much time together since Bobby was born and we really miss you guys. We're getting a baby sitter lined up so hopefully, we can start having "adult" time together again."
Maybe they just assume that you have excluded yourselves from their gatherings by NOT finding a sitter on other occasions. I just recently used SitterCity.com to search for an interview 8 different sitters. I have eased into using a sitter by having one of them care for my sons a few times during the day time, while I run errands, take one of them to the doc appt, or get my hair done. We are looking forward to our first night out alone as a couple in almost 1 year! (We've only had 3 childless evenings since my son was born in Jan 2007).