How do you create handle user roles when users are joined to a company and the roles are company-specific?

Hi, I am creating a web site for corporate users.  The DB structure has users joined to companies in a clean relational manner.  I am using SQL Server for the DB.  Until now, each user has had their own roles, but now I need to change this so that users inherit the permission based on the company's permission setting.  For example, companies will have 'regular' or 'advertiser' accounts (simply a bit field on the company account in SQL Server), and if user belonging to an advertiser company logs in, they should be allowed to add, edit, and delete ads.  I'm using ASP.NET xml site map, breadcrumbs, etc. and storing my user info in SQL Server.  Also, I'm currently relying on ASP.NET sitemap to protect unauthorized users from accessing the wrong parts of the site.
What do you suggest to implement this behavior?  Is there a prefab solution?  Can you create a custom ...