Wide-open writer permissions: can I do that? How?

I want anyone who knows the name of my wiki to have writer permission (because it's for students *and* prospective students, and I won't know who all of the prospective students are before they want to access the wiki, so issuing one-by-one invitations won't work for me).

How do I do that? And if I can't do that, what is the closest I can come to it?
 
sad I’m increasingly boggled that I might have to enter hundreds of users one a time. Tell me that's not the case!
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