Demographics are used to easily refer to a group of people. People are not assigned to a demographic, they instead simply qualify for it based on the demographic's logic.
It is not possible to manually change who qualify for a demographic without altering the demographic's logic or changing the player so they meet the requirements.
Is it not possible to see who qualify for a demographic in v8.3.3, you can only see which demographics you yourself qualify for be looking at the in-game Law window. Demographics you don't qualify fir will be slightly greyed out.
The "Citizen" demographic exist by default on all worlds. It refer to all players that have been online more than 2 hours in the previous 5 days. So if you are online for two hours you will qualify for the Citizen demographic, but if you are barely online or not online at all for 5 days you stop qualifying for the Citizen demographic.
A demographic is created as a law proposal.
From there it has to pass through a voting process. When the proposal is passed it will be added to the list of demographics on the in-game law window.
The demographic has to have passed the voting process before it can be used.
When a demographic is then used in a law or auth of a property or object, it is checked when it is needed. For example; every time you interact with an object that has the a demographic set in its auth, a check will be done to see if you qualify for the demographic.
It is possible to redefine the logic of a demographic.
This is currently bugged in v8.3.3, redefining a demographic does not work.