A private workspace is indeed this: private. No one except the workspace owner has access to it, not even admin.
To let several users work on shared content (resp. grant them READ only access) you can setup a public or collaborative (or confidential) workspace. To grant a user access you must make it a member of that workspace. Only the workspace owner (as well as other users who have WRITE access to the workspace) can create memberships.
When you create user A it will not automatically become a member of any workspace. So the only workspace she can create content in is her private workspace. Every user has a private workspace (and she can create further ones at her need).
As an example take the default workspace “DMX”: it is a public workspace, that is everyone can READ but only members can WRITE. To allow user A to create content in the “DMX” workspace you must make it a member.
Every workspace has one owner and 0 or more members. The owner of a workspace is the user who’ve created it. The owner of the “DMX” workspace is “admin”. Workspace ownership never changes. Workspace memberships do.
And there is indeed one bug: only admins should be able to create user accounts, not every user. Admins are those users which are members of the “Administration” workspace. https://git.dmx.systems/dmx-platform/dmx-platform/issues/242
Information about access permissions and the various workspace sharing modes is in the user guide: https://dmx.readthedocs.io/en/latest/user.html#collaboration-and-sharing
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