An Unincorporated Association (UA) requires at least two legal persons (which can be people or legal entities) to be members. In other words, you need two legal persons to "associate" with each other. So, if we use UAs, we'll need to have at least those two members in the UA, though we could have many more. They could be the Chair and Vice Chair, or they could be two or more of the members of the SO/AC (or even all the members of the SO/AC). I believe we intend to give the SO/ACs fairly broad discretion to establish their UAs as they see fit (including using legal entities other than UAs, such as non-profit corporations or LLCs), while providing them with some high level standards and guidelines so that they work as intended.
On a technical legal level, I don't believe there is a bar to having the Members of ICANN be natural persons (i.e., people) rather than UAs acting as alter egos for the SO/ACs. This creates some secondary issues. Legal entities have Bylaws or similar rules; people don't. So, the behavior of a natural person acting as an alter ego for a SO/AC would have to be regulated entirely by a contract between the SO/AC and the natural person. With the UA, most of the rules about how the UA acts can be embodied in its bylaws, and the contract between the SO/AC and the UA (if one is even needed) would be much simpler. If a natural person is a member, I think the membership would change every time the natural person changed; so you would have to go through a process of members resigning and joining fairly regularly. With the UAs, the membership would remain constant (subject to further changes in ICANN governance and the ICANN community's structures and organizations). Another complication arises in considering how to recall the board; most likely, this would require a contractual agreement among the members to act in concert and have each SO/AC remove the board member(s) that SO/AC appointed. This agreement could remain constant if we use legal entities; if we use natural persons, the agreement will need to be amended and re-executed (at least in part), whenever there is member turnover. Finally, there is just the "optic" of having, e.g., Alan Greenberg as a Member of ICANN, rather than having "ALAC Prime, an Unincorporated Association" as a Member of ICANN.
Greg