How to shoot a party scene with few people?
I agree with @DrMayhem, but I have a few things to add.

You only need one or two establishing shots that show the whole group.  The audience will continue believing that you're in a room full of people after you show it to them once.  For the establishing shot(s), you can shoot the same people, wearing different clothes &amp; accessories, in front of a green screen.  Make sure the lighting and the height of the camera will match with the lighting and camera height in your party setting.  When shooting the group in front of the green screen, position them closer to the camera.  Then shoot the same people again in the actual party setting, farther from the camera.  In post-production, layer the green screen footage of the people over the actual party setting, and it will give the impression of twice as many people.  You can also do 2 "layers" of green screen footage, with the same actors 5-10 feet farther away from the camera, in front of the green screen (and wearing different clothes, hats, glasses, wigs, etc.).

For the rest of the scene, you can use medium and close-up shots.  Include people on the edges of the shot, and passing through the shot, and behind the main actors, to make the audience feel as though there's a whole party going on outside the frame they're looking at.

For the end of the scene, you may want to show the establishing shot (green-screened) again, to "bookend" the scene.

The scene's audio will help "sell" the illusion. Layer additional voices (not more than the number of party-goers!) and various party noises.

I did a cheaper version of the green screen trick in the middle of YouTube video, to make a conference room look full of people even though I only had 3 actors. The establishing shot only lasts for a second, and the action draws the viewer's eye away from the fact that the "people" in the chairs around the table are actually just still pictures I overlaid in post.  Here's the link if you want to see the vid.  The effect shot comes shortly after the 32-second mark, but it's more convincing if you start watching at 0:32 (or at the beginning).