If you just want a document that others can easily edit, a shared Google Drive document might do. Just be wary of people going in and deleting everything (don't give the public full access, only your collaborators).
While asking a similar question myself someone recommended DokuWiki and I've been using it almost exclusively since.
Personally I would recommend downloading the pen drive version and copying it to Dropbox, you could always arrange hosting somewhere but it would be more complex and probably end up costing money (unless you want to simply grant access to everyone and use something like wikia).
In short I agree wikis are a great way to go.
I suggest to use some concurrent versioning system like CVS, SVN or GIT. These are used by software developers to write program texts and are freely and readily available on GitHub, for instance. They already support all needed security and can be quite user friendly (see overview of user friendly interfaces).
They allow to setup the following development process:
Normally there are many pull requests opened and under discussion at the same time.
The most important feature of these versioning systems is that multiple contributors can make changes in parallel inside the same text document, as long as changed parts do not overlap. If one pull request contains rewrites near beginning and another near the end, these can be submitted, discussed and merged or rejected independently, in any sequence.
Merging is only supported for the plain text. Versioning systems also support binary files like images, but cannot merge multiple independent changes, the latest version just overrides all others.
There is no such thing, yet
to even make something like this would require there to be some way to format/organize this. As far as I know Ive never heard of a standardized structure to world building and if there is it must be annoyingly complicated and overly rigid.
Thus at this point you gotta use some kind of generic collaborative document generation tool.......like google docs.
Great thing about google docs is it has version history so if someone flips the table on your project you can just revert their changes.
3.Ability to include texts and images (maps and illustrations)
4.Easy to use
google docs is a google product, just about one of the few companies with a significant track record on ease of use.
5.Somehow secure (not allowing a bad-faith collaborator delete all contributed material)
on google docs you can whitelist your contributors preventing some troll from showing up and wrecking everything. You can also revert changes using the revision history.
and that can give a formatted book as an output
Sure google docs can do that too....though again the formatting is going to be on you.
I only suggest this next thought because it would be such a proper, fun even, way to really collaboratively attack building a world.
Now if you want a real approach to collaboratively building a world. You can repurpose an agile tool like JIRA, plot your high level target world ideals as epics and pose research questions as stories that you pass off to willing team mates that link to a google doc where the answer is built.
This way you organize and distribute your world building in a trackable way. providing this kind of structure would actually stimulate and focus collaborative effort. The down side is if you are unfamiliar with agile and agile tools this would be a pain in the ass to figure out.
GoVisually is a possibility.
You can mark annotations and place comments on any part of your project, and there is this pretty cool feature where you can click on each user and it will hide or display relevant content, so if you think some user is just trolling around you can ban him. Revisions can also be easily upload and it's simple to switch between different versions of the same document or project.
And I'm not entirely sure, but I think it's still free.
Here's a promotional video I found on google: Video