From your admin dashboard....Add your guests email address to your "users", and set the parameters. Admin, Contributor, etc. They will then be able to log in to your admin panel and perform whatever functions you allow them to.
also check "Anyone can register" in Settings > General if you would like to allow anyone visits your blog to register ...
Whats the best to give New User Default Role Subscriber Administrator ( Definitely NO) Editor Author Contributor
If you want the NEW users to register and just post (without restrictions). I'd say Author mode. If you want more control over their actions, I'd say Contributor. Only, admins, editors, authors, and contributors can write posts. If you want more control over what the users can do, then you might want to look into Role Manager related things. http://codex.wordpress.org/Roles_and_Capabilities Huge list of plugins down at bottom of page here: http://codex.wordpress.org/Roles_and_Capabilities#Resources