Hello there, Matt.

You can still use your existing email address to add or create another company. Please know that each company file has its subscription, but you can access them with the same sign-in info by following these steps:

Step 1: Select a subscription you prefer for your new company file by visiting the QuickBooks pricing page or by clicking this link: https://quickbooks.intuit.com/pricing/.

Step 2: If you're already signed in to QuickBooks: 

You can skip this step if you haven't signed recently. 

1. See if the company file that's being shown is the one you want to use.
2. Click Yes, that's correct.
3. Follow the instructions on your screen to create a new company file.

Step 3: If you haven’t signed in recently:

1. You'll be prompted to create an Intuit account. Do not fill out this form unless you want to link your new company file to a new account.
2. Find the Adding a company to an existing account? section. Click the Sign-in link. 
3. Use the user ID and password you already use for QuickBooks for signing in. 
4. Follow the instructions on your screen to create a new company file.

On the other hand, you can also change the primary admin in your existing company file and assign it to your new account. If you can no longer access your old company, you can request to be the primary admin. 

Here's how: 

1. Sign in to QBO as the primary admin. 
2. Go to the Gear icon located at the upper right corner, and then click Manage Users. 
3. Press Add user. 
4. Enter the Name of the new account, and select a role for them by clicking the Role dropdown.
5. Press Send invitation. 

After sending the invitation, click the link and accept the invitation so that your new account can now be the primary admin. 

To learn more about the said steps above, you can use this material since it contains a lot of information that will help you further about adding another company file to your existing email address: 

• Create or add another company file to QuickBooks Online
• Use one user ID to access multiple companies in QuickBooks Online

Moreover, you can also get started and adjust settings after you sign up for QuickBooks Online after you sign in to your QBO account. 

I'm glad to help if you need further assistance or have any additional questions regarding QBO. Just let us know in the comment section. Have a great day!