The PMBOK uses the term 'Supporting Experts' in the list of project team roles, defined as
Supporting experts perform activities required to develop or execute the PM plan. ... such as contracting, financial management, logistics, legal, safety, engineering, test, or quality control ... may be assigned to work full time or may just participate on the team when their particular skills are required.
I don't tend to want to get caught up in definitions or wordsmithing, but 'stakeholder' and 'project staff' or even 'user' are just very clear, broadly used terms that help keep a conversation nice and pithy and keep everyone on the same page with the same vocabulary.
But I find I lack a term for someone on the ground floor that gets brought in occasionally for interviews, to bounce questions off of, or clarify some nuance. As in "John who works in that department and knows the product in and out, but isn't formally on the team, but he is our go-to when making sure we understand something about the customer/product/market."
'Oracle' works behind closed doors, but I want something I can put into an email. "Person familiar with the production" is too hard to say in conversation.
Is 'supporting expert' appropriate for the role/work I've described? Or is there perhaps a better term?