diff -up shadow-4.1.2/src/groupmems.c.checkName shadow-4.1.2/src/groupmems.c
--- shadow-4.1.2/src/groupmems.c.checkName	2008-09-24 11:17:02.000000000 +0200
+++ shadow-4.1.2/src/groupmems.c	2008-09-24 14:03:01.000000000 +0200
@@ -43,6 +43,7 @@
 #include <pwd.h>
 #include "defines.h"
 #include "groupio.h"
+#include "chkname.h"
 
 /* Exit Status Values */
 
@@ -54,6 +55,7 @@
 #define EXIT_NOT_PRIMARY	5	/* not primary owner of group  */
 #define EXIT_NOT_MEMBER		6	/* member of group does not exist */
 #define EXIT_MEMBER_EXISTS	7	/* member of group already exists */
+#define E_BAD_ARG		8       /* invalid argument to option */
 
 #define TRUE 1
 #define FALSE 0
@@ -225,6 +227,12 @@ int main (int argc, char **argv) 
 		usage ();
 	}
 
+        /* check if user names is valid */
+	if ( adduser && !check_user_name (adduser)) {
+	        fprintf (stderr, _("%s is not a valid user name\n"), adduser);
+	        exit (E_BAD_ARG);
+	}
+
 	if (!isroot () && NULL != thisgroup) {
 		fputs (_("Only root can add members to different groups\n"),
 		       stderr);