How can I add another company to a existing QuickBooks online account?

If both of companies are located in the US, you have to open a separate QBO account

https://quickbooks.grsm.io/US

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Welcome to the Quickbooks Community, Paka18081.

You can have multiple companies attached to one QuickBooks Online account. Each company you add will have its own subscription.

To add a new company:

1.  Go to the QuickBooks Page that details the pricing and select the subscription that fits your business best. 

2.  You'll see one of two sign-in pages.  

• If you haven’t signed in recently,  you’ll see a page that asks you to create an Intuit account to buy QuickBooks. Scroll to the bottom and look for the small text that says “Adding a company to an existing account?” Select Sign in next to this. When asked, enter the same ID and password you already use for QuickBooks

• If you recently signed in to QuickBooks, you’ll see a page asking you to confirm the account you want to sign in under. If it’s the account you want to manage the new company with, select “Yes, that’s correct.”

       3. Fill out the information for your new company.

You'll be asked which company you want to open whenever you sign in. If you'd like to switch between companies, Click Settings ⚙️, and select Switch Company.

If you already have another company under a separate QuickBooks Online account, you can easily move them all under one account. 

If you have any questions, please feel free to reach out to me here.