Automatic VPN Connection Methods

I want to get our roaming laptops to connect to the Fortigate VPN when outside the office.  I also want to make it as invisible as possible to the user.  I know the fat FortiClient has an option to enable the VPN from the sign on screen, but that requires the user to select the option.  I don't even want them to have to think about it.  Also, I want to be able to do this without having to purchase a FortiAuthenticator.  We'll only have a max of 6-8 roaming clients connected at a time.  Is anyone else doing this?  What are our options?