The work must be managed in a legal and ethical manner. Some countries are not so diligent in regulating and protecting their archaeological resources. Field workers often start out with a lesser level of training and learn by practical experience under the direction of the senior staff. Volunteers are often welcome on sites to help with the work and to introduce the general population to the methodology and science of archaeology. It is often the thrill of discovery that lures people in the first place and a good field director will recognize the enthusiasm and dedication that novice workers can bring to a project.
Volunteers always have other skills that come in handy.
They may be good at photography, sketching maps or drawing artifacts, small engine repair, cooking, playing the guitar in the evening - all these skills add to the synergy that develops when a group of people get together. It can create a very rewarding experience for everyone involved. A much travelled friend once made the suggestion that has served well over the years. They are big, strong and do not weigh anything. It's great being the only one to produce a napkin at suppertime, a spare piece of toilet paper, a clean absorbent cloth for a scraped knee, a soft and safe wrap for a delicate artifact.