You do not want to expose a Windows share (SMB) to the Internet. This is really dangerous. What you should do instead is create a Virtual Private Network (VPN). A VPN allows remote users to create a secure connection over the internet to your server. The advantage of this is that once connected, the remote users are directly connected to your network. They can access everything (or as little as you allow) inside your network, such as printers, servers, shares, etc.
Here is an article on how to set up VPN access on SBS 2011.