Can I make certain wiki's password protected, so only certain players can access?

Can I make certain forums/journals/wiki's password protected, so only certain players can access?

In my new game "When Autumn Lasts Forever" I want to have areas that only certain players will get access to... Like a wiki or journal which will act as private NPC conversation, info that they know, or their 'secret diary'?

When you edit a wiki, you can change the permissions to 'GM only'. When you do so, there is a list of checkboxes that allow you to open up access to specific players.
Although, I could see adding a feature to unlock wikis with a password. This would allow you to set it up ahead of time and then players could unlock a wiki on their own. Perhaps during a game session, even.

cool thanks!

Will the player be able to post in it?

yes they can... figured it out all by my little self.