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At the moment you cannot create users using the API.
If you have want activity to be recorded about multiple users (as though they were individual users on the web platform) you should build API interactions with every user rather than only having API interactions go via an Organisation Admin user.
However if the integration that you are building does not require that actions be attributed to specific users, you could instead only build API interactions with a single Organisation Admin user
There is no expiry of each authentication token. Each user has a fixed token.