BuddyPress.org

top_level nav
https://codex.buddypress.org/developer-docs/functions/core/bp_core_new_nav_item/

subnav example, add to your plugins/bp-custom.php
https://codex.buddypress.org/extending-buddypress/bp-custom-php/

//generate the 'Bongo' subnav tab to Messages
add_action( 'bp_setup_nav', 'add_subnav_tab', 100 );
function add_subnav_tab() {
global $bp;
bp_core_new_subnav_item( array(
'name'            => __( 'Bongo', 'buddypress' ),
'slug'            => 'bongo',
'parent_url'      => $bp->loggedin_user->domain . $bp->messages->slug . '/',
'parent_slug'     => $bp->messages->slug,
'screen_function' => 'messages_screen_bongo',
'position'        => 40,
'user_has_access' => bp_is_my_profile()
) );
}