@pierre-chopot,
There is no ordering or sorting action on secondary group list at the moment. It's being sorted automatically by MySQL, we assume by group name or group ID.
We've added a hook on the SQL allowing you to change the order once the new version is released. By default we've set it:
The function which responsible for getting the secondary usergroups is located in the wpforo/classes/UserGroups.php file:
function _get_secondary_groups()
Once the next version is released you can use this hook code if the current order is not what you're looking for:
add_filter('wpforo_get_secondary_groups_order_by', function ( $orderby ){
return 'ORDER BY `groupid` ASC';
});