I've had to tackle this very same problem before and unfortunately there's not a whole lot you can do; Depending on the codec and just how far underexposed you are will dictate how far you can push your blacks; In doing so, you will introduce noise, however, I've had varying degrees of success using Magic Bullet's Denoiser, which applies a subtle blur over the entire image. Out of the box it does a decent job, however, you run the risk of blurring out people's facial features on close ups, like eyebrows, if you aren't careful.
In the future this is something you really need to speak with the couple beforehand, as well as with the wedding coordinator in so that they understand that while turning down the lights adds to more of an intimate setting, they run the risk of a dark image during the reception portion of their DVD/Blu-Ray. This is all predicated that you're being paid for the work, of course
Were you using any gain when shooting?