How create a role that only can create users?
Go to admin/people/permissions/roles and create your role. Then go to admin/people/permissions and uncheck all the permission for the new user and check the Administer users permission in the User section:



To uncheck all the permission I can sugest you the use of the Permission Select module.


  Permission Select is a module that allows for quicker editing of role
  permissions on /admin/user/permissions.


And as Pierre Vriens says in his comment: 


  "do not grant ANY permissions to authenticated users". Because any
  user with permission to create users is also an authenticated user.