So what exactly do civilians do while the men re-enact Civil War battles and camp life? There are many options. The first thing to keep in mind is that civilians during the Civil War would not have been present in military camps in the numbers that you commonly see them at re-enactments. That said, the civilians who were in camp usually had a job that connected them to the military for some reason. For example, women who accompanied their husbands into camp would work as a camp laundress or possibly a cook. Officer's wives might sometimes accompany their husbands as a special privilege, but this would definitely not be the case for the common soldier. Another reason for a woman to be in camp is if the army was camped near a town; civilians from the town might come to visit, distribute homemade goodies, etc. They might also be called into service as nurses if a battle occurred nearby (as in the case of Gettysburg and many other battles), although they would be behind the lines, and ideally not in the range of gunfire.
Other civilians chose to portray camp life as an example of how life would be at home. Women cooked, cleaned, worked on sewing projects, and went shopping when they were at home; this can likewise be translated to the types of activities civilians portray at reenactments.
I Have to Wear What?? (or, Ooh, I Get to Wear That?)
We are in the process of getting some loaner clothing made for potential civilian reenactors. While the military has loaner equipment and uniforms for the men, the ladies have some skirts and blouses to use temporarily to see how they like the whole "dress-up" experience, before starting investment in their own wardrobe. Reenacting can be a costly hobby, especially initially. In the era we portray, few people would have gone out and spent a lot of money buying all sorts of new dresses, underpinnings and accessories at once; instead it would be more of a process: you wore a dress till it gave out, then it would be put to use in some other way, to get the most out of the garment. It wouldn't have been discarded immediately in favor of buying something new. In a roundabout way of making this point, do your research, invest in one good, usable and authentic outfit at the start, and you won't regret it later on. You can always later build on this foundation and acquire more as you get a feel for what kind of person you want to portray.
What's authentic? Most people new to the hobby won't automatically know this. Beware of some older costume books; they tend to lump 3 centuries worth of fashion history into 30 pages. Books such as these can't possibly do justice to all the changes in fashion over time. Instead, go back to the source: original images from the time period (photographs, period fashion magazines, paintings). Who Wore What? by Juanita Leisch takes a survey of hundreds of original CDV photographs and makes determinations as to what was fashionable, what wasn't, what age groups wore certain styles or elements, and what are "reenactorisms" (things that reenactors commonly do/wear but aren't necessarily documented to the period). It's a good starting places for a new female reenactor.