Luma curve is usually pretty effective. The general curves controls can also help.
You can try adding a light effect...like, the spotlight effect. I've done this before in After Effects and had some success but never with really badly lit material.
To be honest - and not having seen your footage at this point - if the footage is badly lit, I'd feel comfortable saying that it might not even be "fixable" unfortunately. Usually when there is inadequate light in the original footage, the details are simply not there, and trying to recreate them is generally a tradeoff. You might be able to get the exposure up a bit, but you introduce a LOT of noise in the process that is probably just as distracting as a dimly lit subject.
If the detail is there and a minor tweak is all that is needed, there are many options. It also depends on your format - AVCHD, HDV and other long-GOP 4:2:0 formats are fairly terrible with these sorts of rescue operations. High-end formats are much more malleable.
EDIT - Be sure with curve operations that you remember that your subject is in the low tones - even if it RELATIVELY the lightest object in the picture. The curve levels are dictated by absolute brightness cutoffs, so making adjustments on the high end of the curve would likely solve nothing on dimly lit subjects.
Reading your post I remembered about this from Studiodaily. In the end the author plays with the lens flare effect.