Hey,
In our game, we have a forum icon that all in-game users can tap it and jump into the forum site directly. But these in-game users are visiting the forum as guests. So I am wondering if it is possible to make these in-game users automatically registered as forum members, and they don't need to sign in anymore. I want to make all my in-game users to be the forum members this way. Any suggestions?

It sounds like you're looking for a single sign-on integration, which may require some custom coding.  

You can check with @Stuart Silvester