You don't need to join the Documentation Working Group to help us improve the Joomla! documentation. You don't need to be an expert on writing Joomla! extensions! There are many tasks which will help you learn Joomla! while helping maintain the Tutorial pages. Note that registration on this wiki is entirely separate from registration on our other sites, such as the Joomla! forum.
If you would like to contribute to the Wiki and just don't know where to start, please contact a member of the Documentation Working Group.