To add a new company and its owners first go to the Properties page. On the left side, click on the three dot menu across from Companies, and click New.

This will open a popup window. Here you can fill out all the company information. Make sure to choose the correct Operating Account. If you have not added that account yet, you can go to the Accounting - Accounts page and click the three dot menu and add a New account.
The Email alias is a required field and is easiest just putting in the same name as the company with no spaces. It is an option to use when sending out emails from the system, to come from the company names instead of the property management company name.

Then scroll down on this popup box and Add an owner. You can add as many owners as you'd like, but the ownership percentage needs to add up to 100. At least one owner is required.

Once you are done, just click Save on the bottom right to create that new Company. Each company will also have their own set of books for accounting.