Index: og_mandatory_group.module
===================================================================
--- og_mandatory_group.module	(revision 40)
+++ og_mandatory_group.module	(working copy)
@@ -20,7 +20,9 @@
 function og_mandatory_group_user($op, &$edit, &$account, $category = NULL) {
   switch ($op) {
     case 'insert':
-      if (($group = variable_get('og_mandatory_group', 0)) != 0) {
+    $groups = variable_get('og_mandatory_group',array());
+      if (count($groups) != 0){
+      foreach($groups as $group){
         og_save_subscription($group, $account->uid, array('is_active' => 1));
         // mail the admins
         $node = node_load($group);
@@ -66,6 +68,7 @@
           }
         }
       }
+      }
       break;
   }
 }
@@ -90,7 +93,7 @@
     }
   }
   if (count($options)) {
-    $form['og_mandatory_group'] = array('#type' => 'radios', '#options' => $options, '#default_value' => variable_get('og_mandatory_group', 0));
+    $form['og_mandatory_group'] = array('#type' => 'checkboxes', '#options' => $options, '#default_value' => variable_get('og_mandatory_group', 0));
     return $form;
   }
 }