Yes it's finally official. Dan is back. Chris McKenna is back. I'm in too, as well as Tim Saccardo. Gonna make us some Community this year.— Andy Bobrow (@abobrow) June 10, 2013
They're coming back :
1 - Dan Harmon. Cool.
2 - Chris McKenna, a major writer on American Dad before joining Community when it got its first back-9 order, at episode 1.13 — it turned out to be a back-12 order, season 1 having 25 episodes... good times. He's credited on 7 episodes : 1.16 (Communication Studies), 1.22 (The Art of Discourse), 2.01 (Anthropology 101), 2.09 (Conspiracy Theories and Interior Design), 2.21 (Paradigms of Human Memory), 3.04 (Remedial Chaos Theory) and episode 3.13 (Digital Exploration of Interior Design).
He left the ship when the captain -who had it coming- was thrown to the sharks, after season 3. He had joined The Mindy Project (he's credited on episodes 1.04 & 1.10, Halloween and Mindy's Brother). Now, since the captain had jumped the sharks and found refuge on an island called Pilots Not Picked Up (though he did get his Adult Swim show), his loyal lieutenant has agreed to come back, thankfully (do you see all the amazing episodes he wrote?).
They never left :
1 - Andy Bobrow, credited on episodes 2.04, 2.10, 2.18, 3.02, 3.14, 4.01, 4.10.
2 - Tim Saccardo, script coordinator since season 2, also credited on episodes 3.18 & 4.07.
They left after season 4 :
1 - Megan Ganz, credited on episodes 2.08, 2.16, 3.08, 3.17, 4.02 & 4.13. She joined Modern Family.
2 - Annie Mebane, credited on episodes 3.10, 3.22, 4.05 & 4.12. She joined The Michael J. Fox Show.
3 - Steve Basilone, credited on episodes 3.10, 3.22, 4.05 & 4.12. He joined The Michael J. Fox Show.
4 - Maggie Bandur, credited on episodes 3.03, 4.03 & 4.12. She joined The Michael J. Fox Show.
5 - Ben Wexler, credited on episode 4.04. He joined Arrested Development, and now, you guessed it, he's on The Michael J. Fox Show. Yep, this show will have a strong Community vibe.
Aside from Harmon, McKenna, Bobrow & Saccardo, six new writers have been hired for season 5 :
1 & 2 - Parker Deay and Jordan Blum : both were writers assistants on American Dad (75 episodes for Deay, 60 for Blum), and they co-wrote two episodes, episode 7.05 (Virtual In-Stanity) and episode 8.04 (American Stepdad).
3 - Alex Rubens : writer on the first two seasons of Key & Peele, Comedy Central's sketch series.
4 - Ryan Ridley : writer on Dan Harmon's upcoming Adult Swim animated show, Rick & Morty, he also worked with Harmon on Acceptable TV.
5 - Dan Guterman : writer on a few episodes of The Colbert Report, in The Onion and The New Yorker.
6 - Matt Roller : writer of a bunch of funny tweets and at least this Funny or Die video (couldn't find his Imdb page) :
Sources : Andy Bobrow & Tim Saccardo.