Just like FlightGear itself, the wiki can be considered open-source. Everyone is encouraged to register an account and start contributing!
Maybe you noticed that the article on your favourite aircraft is a bit out of date and you can give it a refresh, or the Howto on making coffee on the 747 needs a cleanup, or the one on Blender lacks a picture, you have found a spam link or simply a typo and want to fix it! All are very valid reasons to join and every little effort helps the maintainers, even just adding the proper template to let them know. Remember to add a meaningful summary message, maintainers do read them!
By the way, do you want to help with regular maintenance? That would give you the opportunity to learn a whole lot on the FlightGear project. In fact, maintaining documentation is the classical way of getting into an open project.
This portal provides links to useful wikitools and articles, ranging from how to write your first article to extensive markup techniques. See also the help pages.
One last note. Remember that any contribution to this wiki is licensed under the GNU GPL version 2