Hoffenheim, which has 3,263 people, probably won't reach the Group Stage. What's the least populous city or town to have a club reach the Group Stage (First Group Stage when there were two)?
It should be Farum, Denmark, home of Nordsjaelland FC. They played CL football 5-6 years ago. Town's population is just under 19k. There's also Lens of France, their population is like 30k. I remember, watching CL magazine as kid, that they were saying how amazing is fact that their stadium is sold out on regular basis although it's capacity is bigger than Lens' population.