Wiki Settings

You can set whether to enable Wiki in the project's settings.
Click 'Settings' on the navigation bar at the top of the project's main screen to move to the project's settings management screen.

Scroll down to the Wiki configuration field and check the checkbox to the left of 'Enable the wiki in (project name)' to use the Wiki.
 
For a contributor who involves in a project to edit the Wiki, first, you need to add the contributor to the project as described in Adding Contributors in this manual.
Next, in the 'Configure' field of the Wiki's settings, set the users who can edit the Wiki. In the screen example, editing the wiki is allowed for the contributors who have a permission to Read and Write.