Adding roles to users
you have to install the Views Bulk Operations and Administration views module. After that, go to the users management interface (admin/people), select the users and select "Change user roles". 

PS 1: You can select users by role (1 on the attached picture)

PS 2: If you need to select ALL of the users, just click the button "Select all xxx rows in this view" (2 on the attached picture)



2/ For selecting only users without role:

You'll have to modify the Views who handle the display:


Go to http:///admin/structure/views/view/admin_views_user/edit/system_1
On the "Filter criteria" section, click on "User:Roles"
Check the "Expose operator" box
Save and go back to the user selection page
Select the 'Only has the Authenticated user role' on the "Role" dropdown


That should really do it now! :)