In most images of people taken indoors, skin lacks important brightness that can prevent it from standing out from a dark background.
Open up an image of a group of people or person. The Background layer needs to be duplicated first of all, which can be done by pressing Cmd/Ctrl+J. Set the blend mode of this duplicate to Screen in the Layers palette.
Screen blending lightens everything in your image, and does a good job on skin tones. So long as there are no bright highlights on the people, the effect should be just right for brightening up faces.
Add a layer mask to the duplicate layer and press Cmd/Ctrl+I to invert it from white to black.
Select the Eraser tool and set its Hardness to 0% and Opacity to 100%. The Foreground colour will need to be black and then you can paint over the skin to reveal the brighter layer underneath.
If the effect is too bright then lower the Opacity of the layer and not the tool.
Here’s the final image with the Layers palette…