A lot of the folks who work with me were dropped overnight into projects they had no clue about, and couldnt get any either , because all the relevant info was with the people who were busy, and no one had any time to transfer the info either. Now that we've learnt the stuff, we've encouraged most of the people to share the info so that others will not have the hard time we had. We use drupal btw.
More often than not, its about moving relevant stuff off people's inboxes , and into a content management system that can be acessed by all, besides , searching in drupal is way faster than a mailbox search.
Of course all the wikis and CMS's would come to nothing if no one believed it. "buy - in" in management speak . you need it at all levels. and it doesnt stop at setting up a wiki, and leaving it at that. The whole thing is , at the risk of sounding cliched, a process, and some incentive for people to do this ( like karma points for adding stuff) is helpful. you need to have people contributing , and recognize and reward the effort , and make people realize the difference it makes to them. If it just makes the job of , say management or IT better, it wont help . You should be able to show that it helps the people who are contributing to it...