I deleted my original post in this thread because I FINALLY FOUND THE WRITTEN GUIDANCE that amends FAA Order 8130.2G!
I originally wrote, "before this program [APP], builders were only permitted to have “required crew” aboard for initial flights, which usually meant that every Phase I E-AB aircraft was legally required to be flown solo." If additional people other than the pilot were needed aboard the aircraft for flight testing, that need would be listed as a part of the Phase I operating limitations. Any other person than the pilot on board an aircraft during Phase I, unless that other person is specifically listed in the operating limitations, is considered a "passenger." Pilots are not allowed to carry "passengers" during Phase I.
Specific guidance for this can be found in
AC 20-27G, page 28, which states under the "restrictions" section that:
(1) Carrying Passengers. You may not carry passengers while you are restricted to the flight test area or during any portion of your phase I flight test program. We suggest you use a tape or video recorder for recording readings and other similar tasks. If you need an additional crewmember for a particular flight test, specify that in your application program letter for the airworthiness certificate. We will list this need in your operating limitations.
So, before the additional pilot program is was theoretically possible to have someone along on certain flights to act as a data recorder or safety pilot of sorts, but to the best of our knowledge it was extremely rare and unusual for builders to ask for additional crewmember language put into their ops limits.
Basically: if more than one person is supposed to be aboard an aircraft during Phase I, the ops limits will reflect that. If they don't, it's supposed to be a solo flight.