How to add another company to my account

Welcome to the Community, mattdoughty. 

Before sharing my insights, may I ask how you would like to add your company to your account?

If it has another entities, you'll need to have a separate subscription for each company file.  However, you can access them all with the same login details, making it easy to switch between them. 

You can access this article for more information about managing multiple company files: Create or add another company.

 In addition, subscriptions in QuickBooks have their usage limits. You can check this article to know more: Learn about usage limits. 

Please comment if you have another question. I'm always here for you. Have a wonderful life. 

@mattdoughty 

One QBO account is for one company file. You can buy QB Desktop Pro Plus to manage multi company files with a single license.