There's probably little value to be gained by bumping this thread, the pointers to the relevant documentation have been provided, but I felt like jumping in and saying that maybe instead of answering directly what Stefan asked for (help getting FTP working) we should emphasize better ways to accomplish his goal? I'm sure we've all been there and "asked the wrong question" for what we really needed to accomplish...
I do see some such discourse above, for example pirateghost pointed out that FTP is horrible and insecure. Frankly I couldn't have chosen two more appropriate words. Nobody should use FTP for anything public-facing IMHO. BTSync and Plex were mentioned as possible alternatives, and I think we should build on those ideas to help the OP find a solution that works for him.
For one, if the FTP usage-model is exactly what his friends need, meaning literally puts and gets on files (why?), then shouldn't we encourage him to use SFTP instead? I'm sure he's already running the SSH service on the FN server, so it should be as simple as exposing the service through his firewall, and of course ensuring it's adequately secured. Not that public-facing SSH is the smartest thing in the world, but if configured with digital certificates instead of brute-forceable passwords, I'd wager it's far safer than an open FTP server...
If literal FTP usage-model isn't required, it opens up a plethora of options. A friend of mine and I used to use a (Windows) peer-to-peer program where we exchanged digital certificates with each other and then shared access to certain directories on our PCs across the country. My shared directory just happened to be a CIFS mount off my Ubuntu server, so Stefan could use something similar to share his FreeNAS datasets securely with friends. The nice thing is that it all looks like local files, drap & drop, etc. As a completely different approach, his friends could tunnel into his LAN and mount shares off the server directly (or run FTP from INSIDE the firewall), no special applications required. There are near-infinite programs and protocols available to accomplish simple file transfers!
So to sum up for the OP, FTP may stand for "File Transfer Protocol", but in the real world, it is quite possibly the absolute worst possible way to "transfer files". If you absolutely must run FTP and only FTP for your friends, then my recommendation is that you're already doing it the "best" possible way, by spinning up a VM to be the FTP server and mounting FN shares read-only. This way when the FTP server gets compromised, the attacker only owns your VM, which you can restore from clean snapshot, and their prize is read-only access to whatever files FN shared, no major harm. If you run the FTP server on FreeNAS itself, all bets are off once it's compromised...
