Since you’re just sharing things within a small group, I think it’s safe to give out your IP or a domain pointing to it. However, you need to put your services behind a DMZ. Make a separate VLAN on your network and put the Pis behind it. Make sure traffic from your private LAN can enter the DMZ but nothing from the DMZ can go to your LAN unless it’s return traffic for a session started in your LAN. If a machine in your DMZ is compromised, it shouldn’t affect your LAN.