diff --git a/ldapcherry/backend/backendLdap.py b/ldapcherry/backend/backendLdap.py
index b180451..1edffbe 100644
--- a/ldapcherry/backend/backendLdap.py
+++ b/ldapcherry/backend/backendLdap.py
@@ -11,6 +11,7 @@ import ldap.modlist as modlist
 import ldap.filter
 import logging
 import ldapcherry.backend
+from sets import Set
 from ldapcherry.exceptions import UserDoesntExist, \
     GroupDoesntExist, \
     UserAlreadyExists
@@ -23,6 +24,21 @@ class CaFileDontExist(Exception):
         self.cafile = cafile
         self.log = "CA file %(cafile)s does not exist" % {'cafile': cafile}
 
+
+class MissingGroupAttr(Exception):
+    def __init__(self, attr):
+        self.attr = attr
+        self.log = "User doesn't have %(attr)s in its attributes" \
+            ", cannot use it to set group" % {'attr': attr}
+
+
+class MultivaluedGroupAttr(Exception):
+    def __init__(self, attr):
+        self.attr = cafile
+        self.log = "User's attribute '%(attr)s' is multivalued" \
+            ", cannot use it to set group" % {'attr': attr}
+
+
 NO_ATTR = 0
 DISPLAYED_ATTRS = 1
 LISTED_ATTRS = 2
@@ -58,10 +74,14 @@ class Backend(ldapcherry.backend.Backend):
         for o in re.split('\W+', self.get_param('objectclasses')):
             self.objectclasses.append(self._str(o))
         self.group_attrs = {}
+        self.group_attrs_keys = Set([])
         for param in config:
             name, sep, group = param.partition('.')
             if name == 'group_attr':
                 self.group_attrs[group] = self.get_param(param)
+                self.group_attrs_keys |= Set(
+                    self._extract_format_keys(self.get_param(param))
+                )
 
         self.attrlist = []
         for a in attrslist:
@@ -135,6 +155,39 @@ class Backend(ldapcherry.backend.Backend):
                 )
         raise
 
+    def _extract_format_keys(self, fmt_string):
+        """Extract the keys of a format string
+        (the 'stuff' in '%(stuff)s'
+        """
+
+        class AccessSaver:
+            def __init__(self):
+                self.keys = []
+
+            def __getitem__(self, key):
+                self.keys.append(key)
+
+        a = AccessSaver()
+        fmt_string % a
+
+        return a.keys
+
+    def _normalize_group_attrs(self, attrs):
+        """Normalize the attributes used to set groups
+        If it's a list of one element, it just become this
+        element.
+        It raises an error if the attribute doesn't exist
+        or if it's multivaluated.
+        """
+        for key in self.group_attrs_keys:
+            pass
+            if key not in attrs:
+                raise MissingGroupAttr(key)
+            if type(attrs[key]) is list and len(attrs[key]) == 1:
+                attrs[key] = attrs[key][0]
+            if type(attrs[key]) is list and len(attrs[key]) != 1:
+                raise MultivaluedGroupAttr(key)
+
     def _connect(self):
         """Initialize an ldap client"""
         ldap_client = ldap.initialize(self.uri)
@@ -392,6 +445,7 @@ class Backend(ldapcherry.backend.Backend):
         dn = tmp[0]
         attrs = tmp[1]
         attrs['dn'] = dn
+        self._normalize_group_attrs(attrs)
         dn = self._str(tmp[0])
         # add user to all groups
         for group in groups:
@@ -447,6 +501,7 @@ class Backend(ldapcherry.backend.Backend):
         dn = tmp[0]
         attrs = tmp[1]
         attrs['dn'] = dn
+        self._normalize_group_attrs(attrs)
         dn = self._str(tmp[0])
         for group in groups:
             group = self._str(group)