You don't need to join the Documentation Working Group to help us improve the Joomla! documentation. You should only register if you want to make changes to the information contained on the Joomla! Docs Wiki. Also 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.