The following are extracts direct from the Order:
" (6) As soon as practicable after the assessment is made or reviewed, the responsible person must record the information prescribed by paragraph (7) where—
(a) he employs five or more employees;
(b) a licence under an enactment is in force in relation to the premises; or
(c) an alterations notice requiring this is in force in relation to the premises."
and from definitions:
""employee" means a person who is or is treated as an employee for the purposes of the Health and Safety at Work etc. Act 1974[8] and related expressions are to be construed accordingly"
With regard to the status of volunteers and the definition of employees I found the following link interesting, and there is a further link within this page to special types of employees which refers to a number of important legal cases.
http://www.hse.gov.uk/enforce/enforcementguide/investigation/status/contract.htmI dont think things are clear cut at all and we will have to await some case law for a determination of when a volunteer is an employee. It appears to be very rarely the case.