According to your usage, you will do quite some shooting in the dark if I'm not wrong, so I think it would be good to get a fast prime lens, such as the 50mm F/1.8 lens. It's great for taking portraits, but you'll need to maintain some gap between your camera and the subject because your camera has a crop sensor and 50mm will translate to 50x1.6= 80mm field of view. This is like a short zoom fixed lens. If not, then you can try the 40mm F/2.8 prime lens, it will be a 64mm lens in terms of field of view.
I have suggested these lenses because of their wide open apertures and not so high price. You'll be able to take great shots in the night and with good bokeh(the blurry background).
*For portraits - 50mm lens
*For occasional portraits and general purposes - 40mm lens
If you have more work during the day, you can try the 18-135mm F/3.5-5.6. It will provide you with some reach as you can zoom in. And as it's range starts at 18mm, you can get some pretty wide shots too(plus you will not need to change your lenses as this covers your current lens range too), for the cramped places of for larger groups of people.
Hope this helps you. Good luck.