You can use the Serenytics REST API to create/delete users and to add/remove those users to groups.
For example, you can automatically create viewers, and add them in a group which has access to a given dashboard. So when these new users log in Serenytics, they can browse this dashboard.
You can also use this API to modify the users' metadata. Users' metadata is used to filter dashboards' content according to the logged users (e.g. to filter a dashboard per user's business unit).
Here is the link to the documentation (with ready-to-use examples in Python) :