SFTP multiple users?

Finally installed Ubuntu 12.0.4, LEMP stack with Wordpress by guides here, but the question how can I actually assign SFTP access not only to custom created user with root privelegies, but also to more than one?

Because when I execute this commands in /var/www for another user, Wordpress forgets access for uploading plugins/etc:

sudo chown www-data:www-data * -R sudo usermod -a -G www-data username

Is it possible may be to create a group of users? Thanks!