You can get the value of the entire MRVS, parse it, and stop the submit if the length of the object is less than 2 (rows)

function onSubmit() {
	var mrvs = g_form.getValue('mrvs_internal_name');
	if(mrvs.length > 2) { //length of the entire MRVS string - native UI returns [] for empty MRVS value which causes a parsing error
		var obj = JSON.parse(mrvs);
        If (obj.length < 2) { //MRVS contains only one row
            alert('Group table must contain more than one row.');
            return false;
        }
    } else { //MRVS does not contain any rows
        alert('Group table must contain at least two rows.');
        return false;
   }
}