In order to use BBB, a person from your group/project must request activation. We have created a form for you that explains all the other points: https://wechange.de/cms/bigbluebutton/
After we have activated BBB for you, it should appear directly in your group/project dashboard. To deactivate it again or change the settings, navigate to the "Embed video conference" item in the group/project settings (requires admin rights).