To play friendless multiplayer, you just have to follow these 3 steps:

1. Choose the prison you want to play friendless multiplayer on.

2. On the character editing screen (the one with the characters lined up in rows), click on the tab in the bottom right.

3. Select Public, and start the prison.

Thanks for reading this very short guide. I know that it doesn't really belong here, but I may or may not decide to combine this into a bigger guide with more tips.