Last weekend i was doing pictures at a private party for some Italian friends.
For making pictures in such parties where there is not enough light its very important to have a good flash with full batteries with you.
Sometimes the autofocus of the camera doesn't work properly because when there is not enough light there is no contrast and the camera cannot focus. Therefore you need to work in manual focus mode.
Sometimes, that is also complicated because you cannot really see through the camera if the image is really focus or not :) . A good trick that i use, is to focus a priory on something that is for example 2m away from my position and then change into manual focus and try always to make the pics of the people that stays at 2m from your position. If you shoot with an aperture 5.6 for example you can be sure that everything will be correctly focus :)
It is also very important to communicate with the people. Many of them will ask you for a picture with their friends some other times you can ask them if they would like to have a picture.
Here some pics from the night :)