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I want to show the following action.
"Administrator removes and adds users to the system"
Since context diagram contains just inputs and outputs, I can add users in the following way:
Administrator-----User registration details---->System(input)
Administrator<----User login details--------System(Output)
But how do I show removed users?
Long time since I've done DFDs, but as I recall, there are no symbols to distinguish an add from a change from a delete. Any process that changes the content of a data store, you draw a line from the process to the data store. The nature of the change might be clear from context, or might need to be explained elsewhere.
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.
You can use a data flow diagram in a specific manner by not just showing the "registration data" flowing from the administrator to the system, but by showing the flow of commands an administrator sends to the system (with the necessary data attached to each command). This leads to
[Administrator] --> (Command for adding a user/registration data attached) ---> [System]
[Administrator] --> (Command for removing a user/user id attached) ---> [System]
