Sometimes you'll get footage that you absolutely have to use but it's far too dark as is.
One quick way to possibly rescue the footage is to double it up.
1. Place your dark footage on one track.
2. Color correct and brighten it as best you can (using some filter like Color Corrector 3 Way). Don't worry if it's not perfect, and don't forget to bump up the saturation if you bump up the mid levels a fair amount. The brighter something is the less saturated it tends to be.
3. Option-Click on the clip in the Timeline. (Option-Clicking will just select the Video portion of a clip with linked Audio) then while keeping Option held down, add Shift. Drag the video clip straight up to the track above it and let go of the mouse. You've duplicated the video portion of the clip atop itself.
4. Right-Click on the top video clip.
5. Select "Composite Mode" then "Add". If your clip now seems WAY too bright don't worry. Next step:
6. Bring the opacity down to about 50%. I like to hit Option+W in the Timeline for Toggle Clip Overlays then drag the line over the clip to around 50%. (If you want to get it exactly at 50% using this method hold down Command while you drag the Opacity Overlay Line down and you'll be able to adjust it in single digit increments, cool huh?)
7. Double-Click (or hit Enter with it highlighted) the top clip in the Timeline to load it into the Viewer. Goto the Color Corrector tab and start adjusting the Mids slider to the left (less bright) until the image in the Canvas is more to your liking.
None of this is set it in stone, so feel free to monkey with the method here all you want. It's the general idea of overlaying a clip atop itself, compositing it, lowering it's opacity and then adjusting it's levels that's important.
Around here, since so much of the footage we've received is incompetently underexposed, we've started to call this restoration method "bottom up pattern telegraphing" just to confuse the clients and not hurt their feelings too badly.