Cannot activate plugin

I am running wpmu 2.8.6, bp 1.1.3 and BP Group Calendar 1.0.3.

When trying to activate the plugin, I get the following error message:

Fatal error: Class 'BP_Groups_Group' not found in /home/dotwebc/public_html/dollhouse.com.br/wp-content/plugins/bp-group-calendar/bp-group-calendar.php on line 129

Any suggestion?

Thanks,