diff -u b/og_context/og_context.module b/og_context/og_context.module
--- b/og_context/og_context.module
+++ b/og_context/og_context.module
@@ -391,12 +391,7 @@
  */
 function og_context_determine_context($group_type, $item = NULL, $check_access = TRUE) {
   // Enable url and node context handlers by default.
-  $defaults = array(
-    'url' => -5,
-    'node' => -4,
-    'file-ajax' => -3,
-    'system-ajax' => -2,
-  );
+  $defaults = array('url' => -5, 'node' => -4);
   if (!$enabled_providers = variable_get('og_context_negotiation_group_context', $defaults)) {
     return;
   }
@@ -423,7 +418,12 @@
  */
 function og_context_determine_context($group_type, $item = NULL, $check_access = TRUE) {
   // Enable url and node context handlers by default.
-  $defaults = array('url' => -5, 'node' => -4);
+  $defaults = array(
+    'url' => -5,
+    'node' => -4,
+    'file-ajax' => -3,
+    'system-ajax' => -2,
+  );
   if (!$enabled_providers = variable_get('og_context_negotiation_group_context', $defaults)) {
     return;
   }
@@ -695,11 +695,6 @@
 
 /**
  * Context handler; Shared between file upload and system ajax calls.
- *
- * This only works for _existing_ entities, not during entity creation.
- *
- * @todo: Extract the logic in og_context_handler_url() for detection of group
- * using entityreference_prepopulate and use that here.
  */
 function og_context_handler_ajax() {
   $item = menu_get_item();
@@ -715,7 +710,73 @@
   }
+
+  $gids = array();
   $entity_type = $form['#entity_type'];
-  if (empty($form_state[$entity_type])) {
-    return;
+  if (!empty($form_state[$entity_type])) {
+    $gids =  _group_context_handler_entity($entity_type, $form_state[$entity_type]);
+    if (empty($gids)) {
+      // _group_context_handler_entity() doesn't always return an array when
+      // empty reset to empty array.
+      $gids = array();
+
+      // @todo: Ideally we would not have to do this and we would just call og_context_handler_url(),
+      //        however, it is not intelligent enough to handle other paths.
+      //        Maybe we should add a new issue for that and abstact this logic
+      //        out in a single place?
+      //        @see og_context_handler_url().
+      list($id, $vid, $bundle) = entity_extract_ids($entity_type, $form_state[$entity_type]);
+      $fields = og_get_group_audience_fields($entity_type, $bundle);
+      foreach ($fields as $field_name => $label) {
+        $field = field_info_field($field_name);
+        $instance = field_info_instance($entity_type, $field_name, $bundle);
+        $parsed_url = drupal_parse_url($form['#action']);
+        if (!empty($parsed_url['query']) && $ids = _og_context_entityreference_prepopulate_get_values_from_url_query($field, $instance, $parsed_url['query'])) {
+          // We need to validate those values ourself, as we called
+          // entityreference_prepopulate_get_values_from_url() directly, bypassing
+          // entityreference_prepopulate_get_values().
+          $target_type = $field['settings']['target_type'];
+          $valid_ids = array();
+          $entities = entity_load($target_type, $ids);
+          foreach ($entities as $id => $entity) {
+            if (!entity_access('view', $target_type, $entity)) {
+              // User can't access entity.
+              continue;
+            }
+
+            if (!og_is_group($target_type, $entity)) {
+              // Entity is not a group.
+              continue;
+            }
+
+            $valid_ids[] = $id;
+          }
+
+          if ($valid_ids) {
+            $gids += array($target_type => array());
+            $gids[$target_type] = array_merge($gids[$target_type], $valid_ids);
+          }
+        }
+      }
+    }
+  }
+
+  return $gids;
+}
+
+/**
+ * Retrieve the group values from the query paramaters for when we are creating
+ * content. Ideally we would just call entityreference_prepopulate_get_values_from_url(),
+ * however, we are unable to do so since it only cares about $_GET.
+ *
+ * @todo Add ticket and patach to entityreference_prepopulate to do this!?
+ * @see entityreference_prepopulate_get_values_from_url()
+ */
+function _og_context_entityreference_prepopulate_get_values_from_url_query($field, $instance, $query = array()) {
+  if (empty($query)) {
+    $query = $_GET;
+  }
+
+  $field_name = $field['field_name'];
+  if (!empty($query[$field_name]) && is_string($query[$field_name])) {
+    return explode(',', $query[$field_name]);
   }
-  return _group_context_handler_entity($entity_type, $form_state[$entity_type]);
 }