User Management not available yet, but coming
Due to high demand this is certainly very much on our roadmap. There is no concrete time of arrival yet, but certainly within this year.
In the meantime though it is no problem to login to an account simultaniously from different devices.
Sellics customers will automatically be informed once this function is made possible.
If you're not a Sellics user yet, feel free to contact us via the chat on our homepage or at firstname.lastname@example.org, so that we can place you on a notification list.t.