What about something like this running in a point wrangle. Fetch all the group names from the intrinsic attribute where they are stored. Initially set all points to be in the "newgroup" group, then loop over the group names. If the point is in the fetched group, remove it from "newgroup."
i@group_newgroup = 1;
string groups[] = detailintrinsic(0, "pointgroups");
foreach (int i; string group; groups) {
int check = inpointgroup(0,group,@ptnum);
if(check==1){i@group_newgroup = 0;}
}