VPN – Client behind router

Hi,

I’m setting up a VPN-server on our SBS2k3 behind a router/firewall. No worries there, everything works fine.
But I’m a little bit lost when it comes to our users. If they are behind a router/firewall will they also have to open up PPTP or is it only on the server-side?
I’m not feeling like going cross country setting it up for them..