webdiplomacy.net

2 questions.

Why do so many games need an Invite code?
How do I get the code?

They are private games. You will also notice the padlock symbol before the name.

You can start your own public game (under "Games" - "Create a New Game", or start a new post in the forum under new games and ask people to sign up, and then have your own private game, or PM some folks you would like to play with and get a private game together that way.

As Doug implied, those are games among friends.