Network drive over the internet

Hi, I'm new to the world of networking. I have a production company and I need a way to map a network drive from my local FreeNAS drive to my employees over the internet. They are not connected to my local network and I want the NAS to be available as a HDD on their computer. Essentially what I want is for them to access all projects files including video clips and documents. I have heard of FTP of course and CIFS (which apparently is not safe to use via the internet). Currently I have a NAS which has FreeNAS installed and connected to my local network via CIFS. I have been looking everywhere but it has been very difficult for me to come up with a solution that would be almost seamless for the rest of the team to access files. Please, if anybody could help, I would very much appreciate it. I need some real in-depth help. Thank you very much.

You might want to look in to setting up an openVPN connection that way you will be secure and you can share to anywhere easily.

do files have to be stored only on the nas or can also be stored on your clients computers?

if not, try synchronizing everything with bittorrent sync (i think now its called just sync). if yes, you need to set up a vpn and put them on the same vpn network as your nas.

i dont have a freenas box, i have a homemade server computer for storage and backup, and a vpn (logmein hamachi) on it as on all of my other computers/laptops, so i can access my files and make backups from everywhere.

but i still use bittorrent sync for sharing over the internet :D. 

I have tried Bit torrent sync, in fact, I am currently using this method to share the files BUT it's just not practical for us since the NAS has Terabytes worth of storage space and I need everyone to have access to all of that data. The problem is that they would also have to have terabytes worth of storage space, meaning we would have to purchase tons of harddrives for each employee. It seems to be too expensive of an option for me. Also, having an OpenVPN would be great but I'm looking more of free solution if there possibly is one. Thanks a lot guys for the replies, really do appreciate it. 

You can do this without 3rd party software just in Windows itself. However as the article suggests this isn't ideal for security reasons. You could improve the security a tiny bit by masking your public IP using a free service like I use no ip for various voice chat and game servers i run for friends. Simply lets them use a instead of my IP address.
here is a link to set-up the VPN.

Once again not super secure, and that is what 3rd party software will make better.

I'd almost recommend looking into a SonicWall Router with built in VPN and then use NETExtender on each users PC. 

Pfsense has a fabulous OpenVPN implementation on it, its got a web GUI, and an easy way to export client configs. I adore it. I'd use that and some SSL certs and encryption. You'd be fine with that, from what I understand OpenVPN is virtually impossible to crack.