how to Add new Role employer also display in url like user/employer?
You can do this by using any of below module :


Registration role module. After register your self by user can directly assign the role.



  Registration role lets an administrator select a role to automatically
  assign to new users. The selected role will be assigned to new
  registrants.



Webform Rules module.



  This module adds rules integration on webforms. Webform Rules makes it
  possible to catch webform submissions by rules and do whatever you'd
  like to do with it.



Auto Assign Role - This module definitely solve your both the issues i.e assigning role after registration &amp; passing role on URL.



  The Auto Assign Role serves three primary purposes :
  
  
  Provide an automatic assignment of roles when a new account is
  created.
  Allow the end user the option of choosing
  their own role or roles when they create their account.
  Provide paths that will trigger a specific role when an account is
  created.
  



For the user/employer url you also do this by using Path Auto module.



  The Pathauto module automatically generates URL/path aliases for
  various kinds of content (nodes, taxonomy terms, users) without
  requiring the user to manually specify the path alias. This allows you
  to have URL aliases like /category/my-node-title instead of /node/123.
  The aliases are based upon a "pattern" system that uses tokens which
  the administrator can change.



Multiforms module.



  The Multiforms module is designed for the collection of data from
  users. It can be used to easily author multi-page forms which are
  published as nodes. The data collected by the forms can then be
  exported. It was originally designed to handle contest.