Basically you need to forward some ports on your router and restrict the port range that the FTP server uses for passive connections.
For example, let's say your FreeNAS box is 192.168.1.6.
On your router, you'd want to forward port TCP 21 to 192.168.1.6. Also forward a given port range (a hundred ports is plenty) for passive mode connections. So, for example, forward ports 5400-5500 to 192.168.1.6. Then within the FreeNAS config pages, set the minimum passive port to 5400 and the maximum passive port to 5500. That should be everything.
Also, you ISP may block port 21, so you may need to forward another port (2121 for example) from your router to port 21 on 192.168.1.6. Of course, your friends will then need to point their FTP clients to port 2121 instead of 21.
One final thing, FTP sends passwords in the clear, so don't reuse a password from another account.