Up to 8 players can play together.
Players can cooperate...or not!
One server and up to 7 clients.
Clients need the server's IP or hostname.
The official Purple Martians server is:
purplemartians.org
Pick one computer to be the server and determine its local IP address.
On the clients, set the server IP to that IP.
On your router, set up a port forwarding rule to allow incoming UDP traffic on port 24785 to be forwarded to your server's local IP address.
Determine your external IP address.
On the remote clients, set the server IP to the external IP address.