With a group account, the Group Owner user can log in to his account, then click Group Admin to allocate storage space and monthly download bytes to sub-users.
There are two ways: (1) Don't allocate storage space to sub-users. The group owner will create folders in his own account and share with sub-users. Sub-users can log in and access folders shared by the group owner. In this case, they use the group owner's storage space and download bytes.
(2) Allocate storage space to sub-users. Note it will reduce the group owner's storage space and download bytes. You can re-allocate storage space and download bytes among group members at any time. If a sub-user is allocated with his own storage space and download bytes, then he can upload / download files in his own My Storage.
In your case, the sub-user has reached his max download bytes for the month. But your group owner still has enough download bytes. You can allocate more download bytes to the sub-user. Again, it will reduce the group owner's storage space and download bytes. Usually you don't want to allocate all available storage space / download bytes to a sub-user.
Please watch this video for very detailed information: