From b2evolution manual

Here are some simple steps to add a new blog to your site:

1. In the backoffice under the Blogs tab click the link at the bottom to create a New blog.
2. Fill out the form for creating a new blog.
3. Click Save.
4. Now the blog is created and administrators have permission to post to it. If you want other users to be able to post to the new blog, then edit the blog and go to the permissions sub tab to set the permissions as you need them.

[edit] Automatic Setup of New Blogs

There is a user-created plugin which is designed to automatically setup a blog when a new user registers.

The homepage is at http://www.fralenuvol.com/albero/servizi/webmaster/scripts/b2evolution_autoblog_plugin.php. You can also track this plugin on the b2evo forums at http://forums.b2evolution.net/viewtopic.php?t=8146.