@awb I can't think of a POSIX utility for this. I wouldn't be surprised if the omission was deliberate; is there anything in POSIX that requires there to be more than one user? (Who has some arbitrary but constant UID.)
That would be a perverse Unix, but the folklore I heard was that POSIX was deliberately designed so that non-Unixes could conform, for US government contracting reasons. (And I believe some non-Unixes once were POSIX certified.)