As educators, we must consider the security of sites in making decisions. There are a number of web-based wiki sites that require a password and a login system to interact with the site. I think it would be worth using these sites particularly with younger students and perhaps older students who are new to using wikis (simply because there would be less of a distraction from outside editors).
With both PBwiki and wikispaces, you can choose whether to mark your wiki as public or private with one click. With PBwiki, asking others to join, simply involves sending them the “join URL”, whereas with Wikispaces, you send your invitees the address of the wiki and they must request permission to join and wait until it is granted.
In his book, Blogs, Wikis, Podcasts and Other Powerful Web Tools for the Classroom, Richardson makes a case for keeping your wiki public, to access collaboration in its purest form. He points out that it might be beneficial having the class as a whole monitoring the content of the wiki, as wikis are easily restored if necessary and that students begin to teach each other when the management of the content is left up to them.