Well, I am not sure if you understand sub-accounts correctly. If you have a group account, and if you are a group admin (or owner), then you can create/add/edit/delete/disable sub-accounts. A sub-account has its own username and password. So you need to logon with sub-user's username and password directly to our website or FTP.
Group members can share files easily using the groupRead and groupWrite folder. They can also share more folders to different users with different levels of permission.