If they are part of your corporate LAN / WAN, they should be able to access just like a local user. If they are external, you have a couple of options:
VPN: you can give them VPN access to your network, then they can access it like a local user.
Expose: You can open TFS to the outside world by allowing the appropriate ports through your firewall.
You can also take a look at the TFS proxies, but to be honest, I haven't done anything about it yet.
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