To add a new business to an existing account, you must check first if that subscription is assigned to You. From the Clients screen, You can click on assign and a fly out will open. 

Once you verified that you are assigned to that subscription, you can click on Cancel.  

Click the 3 dots and a fly out will open, click on Add new business.  

You will be able to fill out the required information and click Save. If you need to assign team members to that new business, you can also do it in that fly out.