Ok, I found my answer:
1. Go to /wp-content/plugins/buddypress/bp-groups.php
2. After line 200, you’ll see that the nav bar items are added for Groups page
3. Then just add something like:
bp_core_new_subnav_item( array( ‘name’ => ‘Your New Tab’, ‘slug’ => ‘your-new-tab’, ‘parent_url’ => $group_link, ‘parent_slug’ => $bp->groups->slug, ‘screen_function’ => ‘your_screen_function’, ‘position’ => 11, ‘item_css_id’ => ‘home’ ) );
You can change ‘position’ integer considering that all other nav bar items have presets values of 10 multiples (10,20,30…)
