I think you're asking two questions here: how to focus in such low light (since you can't see through the viewfinder) and how to light it to get a nice photo. The D3100 has a small (and therefore dimmer) viewfinder which makes it tough to use in low light. Additionally, I suspect you're using a "kit" 18-55mm lens -- good for many uses, but the relatively slow aperture of f4-5.6 doesn't let in much light, and therefore contributes to a dark viewfinder. A 35 or 50mm f1.8 or f1.4 lens would give you a noticeably brighter viewfinder for these conditions, and both 35 and 50mm f1.8 are inexpensive lenses at about $200.
Even with a faster lens, you may still have trouble with autofocus. Manually focusing in low light can be hit-or-miss, but I always have better results than just letting the autofocus rack back and forth and never locking. When using manual focus, if you can pre-focus to an expected distance -- say, five feet out, a distance you know you are likely to be taking photos at -- you can then just fine-tune the focus as needed. Other tips:
Try to find more light. I know it sounds silly, but there are often brighter areas around that you can use to help set focus and metering: white objects, street lights, moon light, lighters -- anything. If you can find something to focus on that is roughly the same distance then you at least have a starting point for manual focus corrections in your desired composition.
Many better lenses include a distance scale to help set focus. Sometimes they're difficult to judge precise focus with, but they are great to get close. If it's too dark to see the distance scale I pull out my phone briefly and used its screen to light the lens to help me get started.
Do you want to use flash at the party in the dark? A simple blast of flash will certainly let you capture the photo, but may ruin the ambiance and create white-faced party goers who are annoyed that you keep blasting their eyes and leaving them to see spots. I would aim to use a high ISO (probably 3200 or 6400, or as high as the camera will go/is acceptable), and that may be complimented with a very low power flash burst to just help light the scene a tiny bit.