Here are a few I remember or were mentioned but never got any screen time.
Garrett Lambert
Starburns
Vicki
Neil
Magnitude (POP POP)
Todd (Army guy with a weird head, decorated soldier)
Officer Cackowski (former security guard at Greendale, police officer, singer at Copera)
Richie & Carl (Greendale school board guys)
Pavel (Abed and Troy's friend, live in the dorm)
Vaughn (Britta and Annie's ex boyfriend, Band lead vocalist, frisbee scholarship)
Quendra (with a QU)
Rachel (Abed's girlfriend)
Rich (Doctor guy, good at pottery and makes Kettle corn in his garage)
Annie Kim (Asian Annie, Daniel Larusso in Karate kid theatre adaptation)
Karl, Lucas, Juergen, and Reinhold (German trios)
Courtney (pretty girl from billiards class, asked our Abed)
White Abed (Abed but employable /s)
Jenny (White Abed's girlfriend)
Koogler (Do I need to explain?)
Buddy (Wanted to join the study group, ended up joining the cool group)
Archie (Rich "kid", community college whale)
Sabrina (Blonde, pretty secretory of the Dean)
Mariah (Pretty librarian, BOOKS)
Professor Michelle (Jeff's ex-girlfriend)
Professor Duncan (Psychologist professor and temporary Save Greendale community member)
Professor Whitman (SIEZE THE DAY, debate club advisor)
Professor Sean Garrity (Professor P. Proferssorson, theater professor)
Professor Kane (Biology Professor)
Professor June (Anthropology 101 professor, tried to strangle Jeff)
Professor Cornwallis (History professor, a little slip up with a coed)
Alan Connor (Partner at Jeff's old firm)
Dean Sprek (city college Dean)
Andre Bennett (Shirley Bennett's husband)
Gobi Nadir (Abed's dad)
Gilbert (Pierce's half brother)
Rick (Subway)
Blade (Britta's ex boyfriend, works at a carnival)
William (Jeff's father)
Willy (Jeff's half brother)
Mysti (Stripper Jeff represented, and the lady Andre cheated with)
Amber (Pierce's ex step daughter)
Mr. Stone (Pierce's lawyer)
Jeremy Simmons (City College's star debator)