To set a default tab on the group page, please go to the BuddyBoss dashboard - Groups - Group Navigation. Please see the image below:


Once clicked, you will be redirected to the Group Navigation page. You will find the group navigation order and choose your preferred default tab. This should set the chosen tab as the default display on all of your groups. Don't forget to click the "Published" button to apply the changes. You may refer to this image: