Readers besides the caller, as hydrated UserInfo objects: a non-empty ID and a UserRoles array
each ([] for a reader with no roles). The caller is always a reader; listing them again, or listing
a reader twice, is harmless. A reader that is null, has no ID or has no UserRoles array fails the search.
The people, other than the caller, who will see a search's results. See SearchParams.Audience.