If you want to purchase multiple licenses under one account go to your account page and select either the monthly or yearly subscription:

Then click the link at the bottom that says "Purchase multiple seats".

You then see an input field that has your current email address already typed in. 

You can add as many email addresses here as you want to purchase a Marvin account for them:

If you don't want to use Marvin under the account that is being billed, you can just remove that email (the first one) and use this account only for billing.

On the bottom left you can see the total price based on the amount of seats you are buying. 

After you entered the credit card info and signed up, the accounts under the emails you typed in will be available to use. 

In your account you can see anytime how many seats you have currently signed up. 

To add or remove seats click on "Manage seats" in your account status.

Then you can simply remove or add new email addresses and click "save".
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The billing will automatically be pro-rated and the new account unlocked.