I was asked about this a couple days ago, and I noticed myself flinch a bit. I hadn't thought much about a non-member advisor role in forming intentional society. I suppose the association in my head is with external boards, where I haven't seen (in my limited but non-zero experience) board members provide net positive value on either corporate or non-profit boards. Boards in general seem to mostly be a quid-pro-quo ecosystem exchanging celebrity name cachet and status-or-virtue signaling. Not that there's anything wrong with alliances or lending social capital per se, just that it's a shell game that doesn't seem serious.
In the case of boards (of directors) specifically, that game of external control can sometimes-to-oftentimes be disastrous. By not being on the inside, they are set up to miss whatever is really going on that's special inside the organization, and kill it by forcing it to whatever conventional path they (the board members) think that others (stockholders or peers) would get least mad at them for doing. This is pretty well established as one of the main reasons why founder-led-and-controlled startups have become more prevalent after so many companies got sold out by their investors via their board seats. It's a bad model, even for public companies, and probably doesn't need more explaining here.
So what about non-stakeholding advisors? Well... done in the right way, this is probably low downside, high upside! Beyond the alliance-signalling factor, I mean. I had been focused on the difficulty in getting good signal from external people on the outside, but Sarah McManus suggested thinking of them as translation helpers and gosh that really reframes it for me. We'll surely have a legibility challenge with the outside world anyway, and if we could get advisors that do grok our workings well enough to help connect our culture to whatever their outside cultural domain is, well that sounds pretty valuable! Plus, allies that you trust can provide some pretty useful advice through the means of their outside perspective - and if misunderstanding is low-cost (in the form of lightly weighting their judgement, or possibly parting ways) then there certainly should be a role for advisors even early on in this mission.