How to set permissions to multi authors?

First, you want to set them up as users. You do this either by allowing them to register on your blog, or by adding them through Users > Authors & Users. When a user is promoted to author, they’ll have certain privileges on your blog based on their user level. A list of what each level is allowed to do can be found here:

http://codex.wordpress.org/User_Levels

Note that WordPress doesn’t offer a multiple-blog feature at this time, so all authors would be posting through the same blog. However, there are ways of managing this:

http://codex.wordpress.org/Plugins/Restriction