What's the best way to set up a temporary group blog?

Both Blogger and Wordpress will allow that function.

It is called “remote publishing”
Personally I have never enabled it and used the feature however I think it will allow multiple users. In the event it will not simply add the other contributors on as authors. Chances are that you will not need that last step though.
(Other blogging services may offer it but those are the only two I have experience with.)

I set up my new Fluther meet up group by going to Yahoo Groups and follow the directions. It works quite well for me.

Peraps Posterous can do what you’re looking for. It’s blogging through email. I suppose though that you’ll always have to mail from the same address, or something. Then again that would be stupid because email addresses are easily spoofed.

Posterous and Tumblr both can do this.
Since this is a temporary blog, Posterous makes this very simple (But I’m still not sure if you need to grant people access to your blog to contribute). I prefer Tumblr because it has a bit more control and anyone can contribute by going to yourblogname.tumblr.com/submit or sending an e-mail to yourblogname@tumblr.com.

Another cool thing you can do if you have GMail is even send emails with your website e-mail. Everyone will see your emails from yourblogname@tumblr.com which is pretty cool.

Thanks everyone for the input. Posterous is working so well for this project. As a bonus, one of the founders cheerfully answers questions. Excellent site!