How much control do subusers have over their forms?
It is up to you. You can grant subusers no, some or full access to each section. There are two types of subusers: standard users and administrators. Administrators can have access to the Users section and manage other users. They can also manage certain groups of forms.
If you want to grant the privileges described above to an administrator, tick the options can manage other users and can manage groups in the Edit User lightbox (or Add User).
Read more about Permissions.