It's quite easy to do I've found from playing around. Here's a guide to do it on Linux.
1. Open your terminal and type *freeciv-server*:
You will get a server made with settings placed. Here you may set your settings, or you can get the GUI on the next steps.
2. Open up the freeciv client. On the "Connect to a Game" option, you set your name and should be able to see the game you made:
3. Other players can join as well, for example, John:
4. From the GUI, if you have more than 4 players, ensure you change that setting in the networking tab for settings:
5. In the internal tab, ensure that you enable turn-blocking game play mode, and you set all players alternate movement:
From there, your game is set, and when you press play you should start getting autosaves show up on the directory where you started your server.
To play your turn, ensure that your name in the settings is equal to the name that you were set as in the game (so, if your name was Frank when setting it up, ensure that it is your name in the network settings), likewise for your friends.
When you complete your turn, save and send to next person in line by email or whatever method you prefer.
Note that this requires a certain level of honour between your friends, as I'm not sure of how to enable the password setting. However, it should not matter with alternate turns, as player blocking plus only sending to the next person in line should make it difficult for someone to take another player.