What are some tips, tricks and things to look out for when photographing dark-skinned people, especially (but not exclusively) when lighter-skinned people are in the same frame?
Most of my shoots occur in Africa and Latin America, so i deal with this all the time. Cameras auto expose for white skin tone. So, in whatever mode you shoot in, use your exposure compensation to add half a stop to one and a half stops over exposure. After you do this a few times, you'll just "see" where it needs to be. If you have a drastic mixture of skin tones, I'd recommend you do a similar method but maybe compensate on the conservative side. If you are shooting raw it is better to overexpose the the white skin a little so the shadows don't get noisy for the darker skin tones.
The best way to do this would be to use a gray card to get the exposure just right. But it does depend somewhat on what you're photographing.
If you're doing just a head shot, use a gray card if you can first, get the exposure right for that famous 18% grey, and then shoot away. If you're taking more of a full body shot, the skin color probably doesn't matter a whole lot, the cloths will occupy significantly more of the photo. Actually, the cloths can be a problem for anyone's skin color.
Likely you will want to do some checking in post and make sure the skin tones are comparable to what they are in reality. But you should be doing this again regardless.
Hope this helps!