diff --git a/api/js/jsapi/egw_user.js b/api/js/jsapi/egw_user.js
index c40cd79637..490a7a56cc 100644
--- a/api/js/jsapi/egw_user.js
+++ b/api/js/jsapi/egw_user.js
@@ -232,6 +232,24 @@ egw.extend('user', egw.MODULE_GLOBAL, function()
 			}
 		},
 
+		/**
+		 * Set account data.  This one can be called from the server to pre-fill the cache.
+		 *
+		 * @param {Array} _data
+		 * @param {String} _field
+		 */
+		set_account_cache: function(_data, _field)
+		{
+			for(var account_id in _data)
+			{
+				if (typeof accountData[account_id] === 'undefined')
+				{
+					accountData[account_id] = {};
+				}
+				accountData[account_id][_field] = _data[account_id];
+			}
+		},
+
 		/**
 		 * Set specified account-data of selected user in an other widget
 		 *
diff --git a/calendar/inc/class.calendar_ui.inc.php b/calendar/inc/class.calendar_ui.inc.php
index d28a201ede..50576ea5fd 100644
--- a/calendar/inc/class.calendar_ui.inc.php
+++ b/calendar/inc/class.calendar_ui.inc.php
@@ -708,6 +708,8 @@ class calendar_ui
 	{
 		if(!$event || !is_array($event)) return false;
 
+		static $sent_groups = array();
+
 		if (!$this->bo->check_perms(Acl::EDIT,$event))
 		{
 			$event['class'] .= 'rowNoEdit ';
@@ -786,6 +788,15 @@ class calendar_ui
 			calendar_so::split_user($uid, $user_type, $user_id);
 			$type_name = lang($this->bo->resources[$user_type]['app']);
 			$event['participant_types'][$type_name ? $type_name : ''][] = $text;
+			if(is_int($uid) && $uid < 0 && !in_array($uid, $sent_groups))
+			{
+				// Make sure group membership info is on the client
+				Api\Json\Response::get()->apply(
+					'egw.set_account_cache', array(
+					array($uid => $GLOBALS['egw']->accounts->members($uid) ),
+					'account_id'
+				));
+			}
 		}
 		$event['date'] = $this->bo->date2string($event['start']);