If you have several companies using RushFiles, you can easily share the different shares with your main user. 

What you need to do:

You will need to share the data shares from your companies with the main user*

You can follow this guide Share files and folders with others

When you share the data share from the companies you are attached to, they will show in the web-client, PC, Mac and mobile devices so you can access all shares with the trouble of loggin out and in with another user. 

* Main user is the user you prefer to use and login with