I'm running WPMU 2.9.2 and BP 188.8.131.52. I just tried installing Group Calendar (1.0.5) and I get the following error:
Fatal error: Class 'BP_Group_Extension' not found in /path/to/my/site/wp-content/plugins/bp-group-calendar/bp-group-calendar.php on line 162
I've deleted files, and re-tried several times, using both "activate site wide" as well as just "activate". I get the same result each time.