Without a certain main character (I guess).

Examples:
Domu
Drunkard Condo Syndrome
The Hour of the Mice
Men's School
A Funeral Procession of K
Doubt (TONOGAI Yoshiki) (sort of)

Thank you

Edit
I decided to loose the request. What I want is basically a story about some bunch of people and their roles are comparably equal.
Preferably not a light manga but something weighty. Comedy and romance preferably in the limit.
Stories with organisations might fit the criteria but I don't want it to be too much into it.

More examples:
Believers (+ closed space)
Himizu (kinda)
Arigatou (kinda)
Nijigahara Holograph

I suppose (haven't read yet):
Uncivilized Planet
Kurosagi Corpse Delivery Service

P.S. How can I edit topic's title? I want it to be "Closed spaces or/and Group of people".

Camelot Garden perhaps.. though there is a main character

Mmm, let's see...

Maybe:
OKADO Tatsuya's manga.
Demian Syndrome
Breakfast Club

Oh, and there's Chokotto Sister.
It's a really good manga.

Hrodulf and Bjornolfr, you will not be forgotten.
[img]http://i411.photobucket.com/albums/pp199/SyberAngelGabrielle/couplesig.jpg[/img]
[color=black]And if the world were black and white,
you would be my rainbow in shades of grey.[/color]

Click 'n Play!

If I had a fantasy self, it'd be a tentacle monster.

mangas with dorms but has a main character:
Love Berrish
Beauty is the Beast
Zig Zag
Hana Kimi
Komatta Toki ni wa Hoshi ni Kike!
Saigo no Seifuku

manga with no particular main character [from what i remember]:
Rolling [highly recommended]
Strawberry Marshmallow [i don't think they're in a dorm though -.-]

Quote from Yenoh

Strawberry Marshmallow [i don't think they're in a dorm though -.-]

They're neighbours.

Hrodulf and Bjornolfr, you will not be forgotten.
[img]http://i411.photobucket.com/albums/pp199/SyberAngelGabrielle/couplesig.jpg[/img]
[color=black]And if the world were black and white,
you would be my rainbow in shades of grey.[/color]

Click 'n Play!

If I had a fantasy self, it'd be a tentacle monster.

Alright. It took me some time to check recommended titles and ones I had on hold.
Unfortunately no satisfaction. No offence to you guys. My bad my bad.

Hmm... this fulfills your requirement of restricted space: Confession (KAWAGUCHI Kaiji), if you don't mind the art. I truly liked it.

These are maybes:

Kayou Gogo 9-ji - Two room-mates with an unusual living arrangement.

Reset - Not literally in a "closed space," but the main events happen in this virtual game and (If i remember correctly) within one specific apartment building.

Route 20 - Post apocalyptic, with a civilization in a protective bubble of some sort.

Blue Heaven - In a cruise ship.

Bradherley no Basha - Events happen in prison. There's also the orphanage aspect of the story.

Houkago Hokenshitsu - Within a boarding school, but there's a twist to it in the end.

I'll looked through my list later and maybe I can come up with more. I don't know if they really fit your criteria, but I picked ones that (are my faves) I thought might interest you (and what I think works... 😕 ).

Oh! Thank you, MasamiAkane

Confession (KAWAGUCHI Kaiji), Reset, Route 20 were on hold. It's time to read them.

Kayou Gogo 9-ji, Blue Heaven, Bradherley no Basha are complete (though I think only Blue Heaven fits).

Houkago Hokenshitsu was recommended already too many times. I'll give it a try.