During my time in the UK military T&E system, the 'crew', other than an additional pilot, were either those civil service scientific officers working on the projects who were used as FT Observers (FT Engineers, more properly, if they had graduated from ETPS or equivalent as FTEs) or other military aircrew (usually Navigators) who had completed the Aerosystems Course.
In my civil tp incarnation, it's again generally been engineers, with some (at least rudimentary) FTO training, who are involved in the projects.
The Flight Test Schedule should stipulate the crewing requirements, bearing in mind the associated risks, and the EASA Permit will be only for those crew essential to the flight.