They can be very helpful because they are more likely to know students. Anonymity often allows students to do things they wouldn't normally do. If a teacher or administrator knows them, they tend to behave in a more appropriate manner. Thus the teachers and administrators should be inside the dance and supervise any activities there. All of these groups must act in concert, and they must be able to communicate with each other in a rapid and efficient manner.
Radios are absolutely crucial for communication.
If your school doesn't have radios for communication purposes, get them as soon as possible. These chaperones perform several functions. They can sell tickets at the doors, provide coat-check services, help serve food if needed, and also provide a little more supervision of students. Their participation in security should not be formal, but any adults present at the dance who know the students are helpful. Just be sure to let your PTA know months in advance, the schedule for your dances and how many chaperones will be needed.