Those who wish to help are assigned to work with an experienced crew. Everyone is trained at the activity they elect. Weedwhackers get trained on the tool of choice. Chain saw operators must be certified through classes offered by the National Park Service. After you gain enough experience, to feel comfortable working on your own, you'll be assigned to your own section if you want that.
Some volunteers choose to work as part of a crew rather than on their own and that's OK also.
There is always plenty for everyone to do.