A BJC needs security to protect the convention goers, their property, the traders' stock outside trading hours, and the registration desk. A site will usually be fenced, except a small number of carefully positioned entrance/exit points, which are badge controlled.
Some aspects of security can be done by volunteers. All attendees should be encouraged to be vigilant for anyone on site without a pass. Some security is far better left to trained and paid security staff. Overnight security is definitely one of those.
Paid security staff are usually responsible for patrolling the site during the night. They will be dealing with people who arrive on site when the reg desk is closed, as well as addressing problems caused by local youths, or indeed by jugglers.
Paid security should conform to BS 7499:2007.
An example of the instructions given to security staff: BJC 2008 Security Procedure.
BJC 2010 hired a security company that specialised in festivals (rather than night clubs or building sites, for example). This meant that they were more familiar with the atmosphere expected at a BJC. They understood how to be understated and friendly, they were comfortable with "alternative" people, and familiar with the kind of teen drinking / drug taking problems that need to be resolved sensitively but firmly and quickly. BJC 2013 had a security firm which had a similar vibe, who got into the convention spirit, without becoming unprofessional. They even cleaned reg desk area each morning! When they were found to be a little lax, the situation was resolved very quickly, with a heartfelt apology to the orgs the next day, and 2 hourly checks by a supervisor the next night. These were not a local company to the convention (they all camped), and would be highly recommended to next years convention organisers, they would definitely be up for doing it again!
Some security situations may require a team member to make a decision. If a paying customer (or trader) is found starting a fight, harassing people or endangering the convention with illegal behaviour, you have to decide whether to throw them off site.
You will need radios for your team. Your security firm may be able to lend/hire you these, and if not, they will probably know where you can hire some MUCH more cheaply than you can manage on your own. Jak Elyard bought 8 decent programmable radios in 2013 which he is willing to hire out to BJCs.
However many radios you need on site, you need twice that number of batteries (or twice that number of radios) so that you can charge one set while using the other.