I have about 7 people and need to give the feel of 20. I'm using a long lens, any tips or solutions?
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 & 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).
I came up with another idea. This works if there is one main character in the scene. Let's call him/her Chris.
1) Start with an exterior shot of the house where the party is. Party sounds emanate from it (muffled of course). It sounds as though there are 20-30 people in the party.
2) Start in one room of the house, with the camera on Chris. Chris walks through the other 5 people in that room who are busy partying, then passes through a doorway.
3) In another room (the kitchen?), Chris walks through 5 "more" people partying. It's actually the same 5 extras.
4) In the final room where the scene will take place, Chris pushes through 5 "more" people partying and finally starts the conversation with the other person in the scene.
In each of those rooms, the party-goers are disguised. They're wearing different costumes, wigs, hats and glasses. Some are on high heels; some are squatting to appear shorter. The camera only catches the sides of faces and the backs of heads, so no full faces are recognizable from shot to shot. And in each room, the audio is layered to sound like there are 4 times as many people in the room.
I have 3 possible options:
You can do this in post-production.
Alternatively, if you can keep a tight shot (ie head and shoulders of one or two foreground individuals) you can use your other cast members in the back/foreground - as layers. As long as you don't stray outside that area, you should be able to imply a few layers of party goers.
And my preferred option - ask some folks to be extras. Round up a bunch of friends/acquaintances who may do it for free/couple of beers/small amounts of money.