Cannot grant access to users to view submissions

Could you describe exactly what pages you want to be accessible? The entire contents of the "Results" tab already has built-in permissions in Webform and you don't need additional modules; you simply need to enable the "access all webform results" permission for certain roles.

How are you defining "categories of webforms submissions"? What makes a category?

Webform has built-in access control hooks to only allow access to the Results tab in certain situations, such as http://drupal.org/project/og_webform allows entire groups of people to be considered "administrators" for a collection of content, but it requires Organic Groups to function. Depending on your use case, OG may be overkill, but I don't know of any other modules that have implemented Webform's access hooks.