How about a six-pack of beer bottles? You could pick your favorite brand and the seventh person could be the bottle-opener.
When getting my costume today I ran across a group of 7 who were going to be Gilligan’s Island
Chess pieces?
I like ** Kat’s** idea.
Sam, Carla, Rebecca, Woody, Cliff, Norm, Frasier
(I never liked Diane much)
Go as FRIENDS… and the seventh will be Marcel.
The patterns book for halloween costumes has christmas at the end. You could go as parts of the nativity.
One Mary
One Joseph
Three Wise Men
One Angel
One Shepard or little drummer boy
and bring along a doll for baby Jesus 
The seven stages of death.
Anger, denial, bargining,… uh and some others,
All wear bridal dresses
and you would be Seven Brides for Seven Brothers.
The Seven Samuri
The Seven virtues uh nice, honest, sleepy…well maybe not
One could be Jason and the rest be victims and have machettes, arrows, knives ect impaling your scantly clad teen-age bods.
Star Trek
Kirk Spock, McCoy, Scotty, Sulu, Checkov, Uhura
Picard, Riker, Data, Crusher, Troi, LaForge, Worf
Sisko, Dax, Bashir, O’Brien, Kira, Worf,Quark
Janeway, Checotay, Kim, Seven of Nine, Torres, Paris, Tuvok
How about the Scooby Gang?
You’ve got Fred, Velma, Daphne, Shaggy… and two people could go as Scooby (one as each half)
Or alternatively, you could get Scooby and Scrappy (arrgh, how I hate that little monster!)
The 7 colors of the rainbow: Red, Orange, Yellow, Green, Blue, Indigo and Violet.
Get cheap white clothes; pants and t-shirts and dye them the appropriate color. Do body/face paints for your exposed skin. Wear black shoes/sneakers (or get cheap white sneakers and dye them, too).
The 1927 New York Yankee ‘Murderers Row’ line-up.
How about the " saved by the bell " group ? Let’s see you have Zack, A.C. , Screech, kelly , Lisa , Jessie , and Mr. Belding
Six of you dress like dalmations, one Cruella deVille
6 demons, one Satan.
Then how about the seven virtues (thanks, Zebra!):
Prudence (Caution sign)
Fortitude (carry weight)
Temperance (carry milk)
Justice (robe and scales; judge’s robe)
Faith
Charity (big heart)
Hope
OK, maybe not 
ha! gigi, funny suggestion! How’s milk representative of temperance?
Well, it’s going to be Clue characters. Folks have already gotten some of their costumes done. I’m going to be Prof. Plum in the conservatory with the candlestick!
Milk is, like, non-alcoholic?
Seven dwarf-lords in their halls of stone.
The seven tribes of Israel.
A gaggle of geese.
The greek modes (Ionian, Dorian, Phrygian, Lydian, Mixolydian, Aeolian, and Locrian. I’d wanna be Locrian).
Or Greek statues (paint yourself pasty white, wear togas, and strike lots of poses).
A basketball team, a coach, and a referee. Or cheerleaders.
Seven lords a-leaping.
Have everybody wear a big arrow on their chest, pointing up. Tell people you need one more, and you’ll be able to make Chaos.
Archetypal D&D party: 2 fighters, Paladin, 1 clerics, 2 mages, and a whiny thief (but I don’t WANNA listen at the door! YOU try it!)
Synchronized swimmers.
A ballet troupe.
The weasel patrol from Who Framed Roger Rabbit? Sure, there were only four, but who remembers that?
The seven seas?
Seven of the characters from Cantabury Tales?: Wife of Bath, Nun, Nun’s Priest, Friar, Carpenter, the Squire, Prioress, Merchant, Franklin, Man of Law, Cook, Ship Man, Physician, Pardoner, Miller, etc.
You could do them traditional or modern - could be fun.
That’s not how I remember it:
But that’s only six. Somebody has to dress up as a baby (not necessarily invisible pink) unicorn…
… or maybe even Dungeonmaster himself?
Dress all in matching green outfits and call yourselves “Gang Green.”
A hysterically funny batch of Hallowe’en revellers I one saw were dressed as the Von Trapp Family. They’d made lederhosen with drapery fabric that looked way too close to the outfits in the movie, and they sang songs (badly) from The Sound of Music