

Provide them with VPN access. If that’s too much for them, then they don’t get access. Tough. On the scale of security vs convenience, that’s nothing.
If you really really want, you should at least see if you can put a WAF in front, and put the server itself somewhere it doesn’t have access to the rest of your network (a DMZ) so that if and when it gets hacked, it doesn’t compromise the entire network.
It depends on what you’re trying to do with it. Typically people only use Macs as servers when they’re doing development for Apple products.