how to get current user name by user_id in buddypress?

You can use the very same get_userdata function of WordPress to code a specific function. Stick this in your functions.php:

function get_display_name($user_id) {
    if (!$user = get_userdata($user_id))
        return false;
    return $user->data->display_name;
}

So you can do something like:

$display_name = get_display_name($some_user_id);
echo $display_name;