My b2evolution Version: Not Entered

About private blogs, I found only this.

http://forums.b2evolution.net/viewtopic.php?t=12864&highlight=private&sid=3d0850a96bf9f2f4ef2d3a4d952fd156

which suggests asking authentication for the whole site.
What I want is to reset default value of post permission for a particular blog (from public to restricted).

I guess I have to modify a file for this...
but which one shall I look into?

thanx