I say you plant them outdoors, if the weather permits, in shallow graves and have them emerge to walk among the living at opportune times.
Or, have them conceal themselves as a piece of furniture or other decoration, and try to blend in with the rest of the party. See if anyone notices. Likely everyone will feel as though something is ‘just not right’ but might not be able to put their finger on it.
Any cemetaries near you?? This is little easier around bigger cities, because there are many cemetaries. But here’s another one you can try:
Drive around to various cemetaries and check them out around 4-5pm or so. Look around for a fresh dug hole. No one is going to have a ceremony that late, so any hole you find will be there that night. Put a ladder or some boxes in the hole so you can stand on it later.
Sometime during the night, mention how cool it would be to go walk around the cemetary. Get enough people interested and the whole lot of you head out there.
Have two or three people leave about an hour ahead of time. They’ll climb in the hole, stand on the ladder and cover the whole thing with a brown blanket or something. Don’t worry, it will be very dark and hard to tell what’s what anyway. So it doesn’t have to be perfect. Maybe someone can go with them who is not going to hide in the hole. Someone who can make sure they are well covered and he can go hide in the bushes. Maybe hide with an IR capable video camera to capture it all on film.
When the party arrives, all the girls will be huddled together and freaking out. They’ll all be walking slowly and screaming at every cracking twig. (Ever walk in a cemetary at night with girls???)
Anyway, when they get close to the hole, be like “Oh Crap. What is that??”
“What?!”
At this point they look over to see a hand dart up through the blanket, seemingly through the ground!!!
Girls and guys alike will run screaming and pissing themselves all the way back to the cars!!!
Those who are brave enough to still be there. Or maybe not brave, maybe they are standing there frozen with fear and pissing themselves where they stand. Those people will be around to witness the zombies crawl moaningly out of the hole!!!
They will not be standing around much after that! (Oh yeah, don’t forget to dress up as zombies. Though, really, it’s not that important. As long as a hand darts out of the ground and starts moving around!!!)
Make sure you bring a change of clothes for everyone. Because they will shit their pants!
Don’t worry so much on realizism. You are in a cemetary for crying out loud. Peoples’ wild and racing imaginations will supply all the realism.
This one time, I was walking in a cemetary downtown with some chicks. And in front of the was this large mass. It was dark and hard to make out. I walked very slowly up to it. With each step I starred harder and squinted to try to make sense of it. What was it? I got closer and closer and still could not be sure. The closer I got the more it started to take the shape of a man. My imagination told me it was so many things. A dead man. A zombie. A body that animals dug up and dragged around. What was it. OMG, it’s definitely a person. . .
When I got within about 5 meters from it, and just as sure as anything that it was or was once a person. It jumped up and screamed “DEAD MAN!!!” “Ha ha ha ha ha ha”
Fucking bums!
It was pretty cool though. The three girls took off running. I jumped back and almost shit myself. He was pretty cool though. Just looking for a place to sleep before heading to the port in the morning. He didn’t even ask me for spare change or anything…
It would be pretty easy to conceal a real limb and rip off a fake one. Just need to find a convincing fake and stage it so nobody notices the limb doesn’t move.
If nobody notices, then maybe the chair could grab whoever is sitting in it. 
I’m liking Jet Girl’s second idea. The alien pregnancy one seems difficult to orchestrate.
A plague of ants unto the fruit salad, perhaps? 
Sort of like that Starburst (I think) commercial where people are camoflaged as various office furniture, then burst out to steal the Starbursts and run away.
This idea could be great fun without the violence angle. Have 2 actors be the “couple” and have them take the argument all over the party…involving other people, asking them to mediate their outrageous problems (whether or not to use viagra, who lost the damn vibrator, fighting over who gets to be the “S” and who gets to be the “M”, asking “he said I was…fat/flat chested/loud and obnoxious…what do you think?” Just some fast ideas. Your friend could tailor it to her friends. Even plant some other guests for them to interact with…it could be a fun all night long prank. They could even get really wierd about the food, about hating Halloween…could take it a lot of different ways.
Could even involve pie-in-face before the eve is over
On Halloween a few years back we made a straw man and put him in the front porch swing a couple weeks early. All the neighborhood kids and adults alike had seen him. Of course on Haloween night, I swapped clothes with the straw man, straw still protruding from everywhere, and sat in his place on the front porch swing. Worked really well. Scared young and old.
Would a terrorist attack be overboard ?
I was going to suggest this exact gag. You could do the same with multiple dummies/actors; set up a graveyard with dummy zombies, only to have them “come to life” the night of the party.
I like the disturbance idea – the domestic quarrel looks good, because you can turn it around on the prankster – a second set of thespians could be cops responding to the incident, or maybe the girl’s big brother crashing the party – “Hey! Who’s this guy? Does your boyfriend know you’re screwing around?” A fight begins…
Simple, not scary, but entertaining: Saran-wrap a doorway. Take one big long sheet, pull it real tight over the frame so you don’t see any wrinkles and tape it down. Watching someone try to walk through will be just about the funniest thing you’ll ever see.
I grew up in a house with a sliding glass door. I can’t tell you how many times that friggin screen was walked through. Always funny as hell to me but Dad never laughed.
How about a terrible accident. Someone falls in the bathroom (surely you can round up a shatter box), or maybe they are slicing a pumpkin in the kitchen with a really large knife, some beef tripe and blood hanging from the abdominal region… could work.
Can any of your actor friends do impressions? Splice videotape? Tape a lot of generic news (and other) footage, have your friends supply a voiceover, and you’ve got your own homemade War of the Worlds broadcast. It can be about anything – war, disaster, axe murderers on the loose, or the traditional alien invasion. If it’s well-written, your guests may not catch on for a while and it’ll be fun even after they do. Awfully time-intensive, though.
That is a great idea !~! I like it, I like it !! With all of the terrorist footage floating around I could make a full scale invasion !! Forget splicing tape do it all on disc !!
I like the “horrible accident” ones. Someone stumbles in with a bloody towel around his hand, “Jesus I think I cut my finger off!” At some opportune moment (an especially good one is if the mark asks to look at the cut or if the “victim” asks the mark to look at it), when the towel is moved, the “victim” squeezes forth a huge jet of blood from the syringe/tube concealed in the towel.
Especially fun at pumpkin carvings.
You have several actors at the party.
You have one actor that isn’t at the party.
He is the next door neighbor.
He comes over a few times complaining about the noise.
Of course the noise doesn’t stop.
He comes back and shoots up the party! Killing several guests.
Or decorate the house with lots of store dummies. Have a few of the decorations be live people pretending to be dummies. Maybe have a dummy and a person dressed the same and off in a seperate room. The live person steps in for the dummy, does something to feak out a person or two in the room, when they bring people back it’s only a dummy again.
Repeat till someone has a nervous breakdown.
Saran-wrap over the toilet bowl. Now that’s funny!
Well, if it’s not your house and/or you don’t have to clean the bathroom…
I still like the idea of someone getting shot. Of course, someone else at the party will pull out a gun and shoot the shooter. Because you’ve planned it that way, you see…