Irene Molly (Hunter) hasn't done any TV since Andy Richter Controls the Universe, no film roles either. She is into music though. Not sure if she has a CD or has put a band together, whether she's a solo artist...
Lindsay Sloane (Marcy) is still busy with stuff.
Al Santos has got a couple horror movies coming out this year. One of them looks like it might be theatre-worthy, so maybe they're banking on that tie-in for Grosse Pointe
Bonnie Somerville (Courtney) has had arguably the most successful post-Grosse Pointe career, what with a few small roles in major feature films, a recurring guest spot on The O.C. in its first season, as a main cast member on NYPD Blue in its final season last year, and on the recently cancelled Fox series Kitchen Confidential. She also contributed a song to the Zach Braff/Natalie Portman/Peter Sarsgaard film Garden State.
Kohl Sudduth (Quentin) has played a supporting character in a series of Tom Selleck movies these past couple years.
Kyle Howard (Dave The Stand-In) has kept very busy since Grosse Pointe went off the air in 2001. He's had a bit on Friends, was in the Colin Hanks/Jack Black flick Orange County, had a 3-episode recurring guest spot on Boston Public, two episodes of Providence, two for What I Like About You, four for The Drew Carey Show, was a regular on the one-season WB sitcom Run of the House, one episode of Nip/Tuck, a bunch more one-time TV show guest spots, and is currently a recurring guest star on the WB sitcom Related.
I'm only basing all this on a quick IMDB search (the actors' profiles/resumes could be incomplete due to it being dependant on fans submitting info) and a visit to some of their official sites/message boards. Some of them might have done/might be doing indie films that aren't listed, many of them are probably doing stage work, and it's possible some may've moved on from acting (I didn't bother looking up the other three principles like the guy who played Kevin, the PA who kept getting dumped on).