One useful distinction when it comes to planning field work is to think in terms of core and extended team members.
Core team members are usually there from early on in the study; they need to know how the field research fits into the corporate big picture; can multi-task across roles; and will be part of the discussion that shapes the moral baseline. Often they are asked to join all of the study locations. The core team is typically made up of colleagues plus one fixer per study location. (More on the optimal team size here).
Extended team members are hired as needed: to assist in particular tasks; join for a limited period of time or to work in a particular country or city. They need enough information to get the job done, but apart from that can operate on a need to know basis.
All externals – whether core or extended team members from outside the company will be asked to sign an NDA. When researching particularly sensitive topics, for example working on the road map of future service concepts, core team members are likely to be present, extended team members not – if for the simple reason that the level of trust and understanding of the implications of screwing up will be better understood.