When you install b2evo from scratch, there are 2 users created for you.
The admin, and a Mr Demo.
The admin is.. well.. the admin..

But what about letting other users register on your blog ?
Your spouce, friend(s), all the people in your class/workplace...

How to make it that they can actually post ?

Let them register, of register them yourself.
Decide in with blog you want your new members can post.
(Yes, you have to grant them rights for every blog that you have. By default they have no rights at all, and they will continue seeing this message : Since you're a newcomer, you'll have to wait for an admin to authorize you to post. You can also e-mail the admin to ask for a promotion. When you're promoted, just reload this page and you'll be able to blog.)

In the backoffice click on the tab 'Blogs'
Choose the blog you want to alter
Click on the tab 'permissions'
Check the permissions you want the user to have.
Click Save.