Once someone has registered they will activate their account by clicking on a link from their email account.
You can manage them in the User Manager section
I have made an account called test.
You can control their access by clicking on the button and then use Select Group for their access. The default is that they can access but not make any changes but you can allocate, authors, editors, back-end development etc.
In the Menu Manager you can control if guests have access or not.
Click on the Access Level for that button and it will change from Public (anyone) to Registered, or Special.