I would recommend watching this support video on YouTube: How to setup Cloud FTP Server Instantly
The key part to understand is: with a group account, you have two options in allocating storage space among sub-users.
(1) Don't allocate storage to sub-users. You will create folders in your main account, then share them with sub-users. Your sub-users will have no storage space in their own accounts. But they can access folders you shared with them (using your account's storage space and download bytes).
(2) Allocate storage space to sub-users. Your sub-users will have their own storage space. But it will lower your main account's storage space. For example, if your group account has 500GB of storage space, and if you allocated 200GB to a sub-user, it will lower your main account storage space to 300GB.
You can re-allocate storage space among sub-users at any time using the Group Admin feature. I have re-allocated storage space for you this time. You can do so later yourself. Please let us know if you have any more questions.