Add wiki to WordPress?

Is it possible to add a wiki to WordPress?
http://rtoa.us/wp1
I am planning to start a community for reading teachers of a certain reading series. I would like to be able to have members be able to edit certain added pages to my site w/their lesson plans and add resources- I’m looking @ a wiki as a possible solution?
If adding a wiki isn’t possible- are there other addons/plugins that would offer these things?
ThanX~~~ Sharnon