How can my friend connect to my VPN?

Me and my friend work on allot of video projects with eachother, but it takes a long time to drive external hard drives back and forth between eachothers houses... so i set up a computer as a NAS server and set up VPN on it and am having trouble figuring out how my friends are suposed to connect to it. Any help? (My normal IP adress dosint work) i made shur everything was port forwarded.

Some ISPs may block VPN ports for residential customers. Being PPTP or L2TP. I don't believe this is the case though.

Can you give me more information about what kind of VPN you set up, and what NAS it is?

Set himup a FTP/SFTP server  

Just a windows XP NAS server :) and im kinda new to vpn, can you help me out a bit?