You can make products and/or categories exclusive to certain companies - here's how.
You'll see as you're editing a product (or category) that there's a setting for exclusivity, this is where you can set the item to only be visible to a certain company or a group of companies (by tags):
Using tags, your settings would look something like this, then you would make sure all the company who are allowed to see this product have that tag!
Some companies only want certain customers to have access to one (or several) products, but not the entire catalog. Our tag system works the other direction (creating company exclusive products) but not typically excluding products from companies. There is; however, a work around: You would create a tag, maybe "exclude_company_x", and assign it to EVERY product, except the one you want Company X to have access to, and also assigning it to EVERY company, except Company X. You would have to repeat with every new product and company.