There are two ways to add a sub-account to your Parent account. The first way, you can simply go up and click on your account logo in the right hand upper corner and click "Add new account". Hereafter you will be redirected where you need to:
Write in:
Account Name
Account Owner
Industry as well as
Company Size
Click "Save"
and bibbidibabbidibo you created a sub-account where you can add users and create Campaigns 😄🌟
The other way to create your sub-account is to go under "Settings" and then:
Clicking on "Accounts" in your top menu
Go down and click "+ Add new account"
Then you go through the same steps as above and add
Account Name
Account Owner
Industry as well as
Company Size
Click "Save"
Aaand your DONE 🚀