Create one role per company and assign those roles to the users as required.

Company=Role

Then apply view and edit permissions for those roles to your pages and modules. When a user signs in, they will only see content that they have rights to see.

What exact modules you need will depend on what your users need to do.

Rob

How you organize/make data avaiable to users.  Is each company data setup/isolated in a portal (ex. child portal)?  Or you have a single portal and a custom module that will display data authorized to the user?

Break down the logic in two steps

Step 1.  Authentication - login and validate username/password.  If authenticated (valid username/password) then redirect users to a page where users have option to pick next action.  DNN CORE supports page redirect after login

Step 2.  Authorization - what data can the user see?  Per Rob's suggestion, you assign users to DNN security role(s).   To make it clean, leverage Role Group in DNN to cleanly separate custom roles for Company vs. the DNN built-in roles.  It is possible to write SQL query to get a list of roles (or companies) user belongs to.  If there is only one company, show data for that one company.  If more then ones, prompt the user to select a company.  Suggested modules:  NukeDK or IndooGrid 2.0.  You can get IndooGrid and try out for 30 days.