Oh so you are talking about remote control. There are many systems out there. I recommend VNC. It works with Linux and Windows. It gives your remote control of the server. Again you still need to get them connected to your network and that is were VPN comes in. It connects your private network to their private network so that you can talk to all of the computers on your private network. VPN opens a secure connection that is encrypted so that no one on the internet can see it except for those you give the passwords to. Once they connect to your network then they can remote into the server with VNC and there you go.
http://www.youtube.com/watch?v=sGYIsJUIxH8
http://www.youtube.com/watch?v=rFg7TSwVcL4
http://www.youtube.com/watch?v=1La4N_u6WN0
If you just want to share files with them and not necessarily need them to remote to your server then I would suggest setting up a FTP site.
http://www.wikihow.com/Set-up-an-FTP-Server-in-Ubuntu-Linux