In Moodle 1.9, that string was used in moodlelib.
In Moodle 2.x, that string is still present in lang/en/group.php but it is not used anywhere in moodle core. I expect the reason is that modules are expected to test group membership in such ways that the 'groupmembersonlyerror' warning is no longer necessary.
Can developers confirm my findings? Do any add-ons developers actually use that string?
Joseph