In order to get your Facebook Group ID you need to go the group page, for example: https://www.facebook.com/groups/allforjoomla.apps/ and view the html source (Ctrl+U on Windows and Cmd+Opt+U on Mac)
In the html source you need to find fb://group/
It will look like fb://group/2307269526220118 - those numbers are your Facebook Group ID. In this case, the ID for the "allforjoomla.apps" group is: 2307269526220118