The word is not perfectly precise, but it’s meaning is anchored in trial testimony. A witness is someone who testifies at trial, based on their first hand knowledge of the facts of the case. Because there is generally an extended pre-trial phase, where prospective witnesses may be interviewed or deposed, those folks are generally also called witnesses in that context, and the Fifth Amendment extends to such pre-trial proceedings. But all of that is in connection with their possible testimony at trial.

You would not call the IT guy a witness, because “providing access to information” is not “testifying.” A witness is someone who has facts relevant to the charges, and who testifies those facts to the jury based on her personal knowledge. The IT guy has no personal knowledge of the crime and won’t be called to tell the jury about it.

A good litmus test is: would you possibly tell this information to the jury? You may need to know the accused’s office number to search it. But that office number isn’t relevant information to the case in and of itself. It wouldn’t be a point you’d tell the jury. And the receptionist that tells you it isn’t a witness.