DFDs represent flows of data between data stores or dataprocessing functions and external entities (aka sources/sink or input/output). The usual notation, be it Yourdon/deMarco or Gane&Saron show the data as labels on flow arrows.
Context diagrams is a subset of DFD, giving a macroscopic view on a system as a whole, i.e. the system (top-level dataprocessing function) in the center, with the data exchange with the external world (only external entities, in principle no data store).
In your description I see only one external entity involved, the administrator, and one top level function "user registration system" and 1 user flow in each direction.
Unfortunately, this would not show the add and remove. I think you could also have 3 flows to show distinguish the flow from admin to system into users-to-add and users-to-delete.
Remark: I think that you should not show the system as input or output, unless you would have a user management system which is connected to other systems.