I love the idea too. I've just created a new blog site and left it open for others to join, wellness-serenity.joeuser.com. I had a hard time with it too and I did try to create it a long time before this.
Anyway, this is what I did, I'll try to remember step by step:
I went into my blog
Then I clicked on “My Account”
Then I selected from the “My Site” box on the right side of your screen
Choose Select Site Themes and Titles and a screen will open up
Here you can type in the information about your site, the name/title, site description,
There’s several subject information to look at and select what you want, this include the color scheme and whether you want your info to appear on the left or right side of the screen
Select submit
Then wait for it to register and I believe you will be requested to give a name that will be your JoeUser address for that site.
You'll be able to choose whether the Site will be open or close, or whether people can choose to join your blog group
I had to play with it for a good hour the day I made my new site because of computer problems that day I believe so hopefully it won't take you as long. But I hope this helps. I don't remember exact, and I just tried to repeat the steps and changed the wording for my Serenity site! So, try it and see if it works. Good luck!