Who may act on behalf of a newsletter: its owner, plus the members the owner added as admins, editors or guests.
Both sides of that edge are here. A newsletter's roster answers "who is on
this team" and needs settings. GET /memberships answers "which teams
is this account on", which is the same membership read from the person
rather than from the newsletter, and needs account: read instead:
the set of teams somebody is on is a fact about them.