Are they being paid by the venue, as self-employed or employed?
As far as I know, and I'm not an employer, there is no law saying you must go through a company, but there may be tax/ NI issues if you don't. Also there needs to be public liability insurance somewhere to cover your actions. Possibly its covered by the venue's insurance, but it may not be. Its also possible that the local licensing rules stipulate a company needs to be used.
Ask the police what exactly says you need to have a company.
I should add to that, if they are being employed in a licensed venue, whoever employs them also needs an SIA licence. If its a company or the venue that includes all directors of the company and any management that actually manage the DS.