Hi,
the jBackend user module included in the installation package replicates the behavior of the frontend user component / module, so it assigns the user to the "Registered" group (or the one defined in the backend settings).

To get what you need it requires a little customization to this module, or to use an "admin" user module. The latter solution is feasible only for backend to backend integration (I don't know if this is your case).

Kind regards
Luigi