Any GroupRef. Bare contacts labels are normalized to
contacts:<viewerPlayerId>:<label> at the verb boundary; the two
reserved pseudo-subjects everyone-else / strangers are stored as
their bare reserved identifier (they are not GroupApi refs).
A NotifyRule resolved against a
(viewer, person)pair — the matched rule plus whether it came from the virtual reserved baseline (reserved: true) rather than a stored, player-authored row.