I think the most reasonable life drawing classes I've come across are hosted by the Art Students League of New York
, near Columbus Circle. If you've got the time, your best bet to get more info is check out their site, or swing by and pick up a catalog. I know of a few others, but their daily class option seemed great for me during the summer break since I'd be able to get around 60hrs of drawing instruction in per month.
There are more options that the one I gave above, but it helps to know exactly what you're aiming for. You want classes, or just groups that get together and sketch/draw, and whatnot.