You are probably wondering: why does Atlassian not give me the ability to create users via my add-on and will they ever implement that in the future?
The answer is probably: Atlassian will never implement this.
The number of users in an Atlassian Cloud instance is what Atlassian charges customers for. Add-ons are also paid by the number of users that can use the add-on.
Atlassian promises our customers that their ‘site-admins’ are the ones that are in control of the Bill. They decide how many users and add-ons that they have installed, and thus how much they pay every month. Allowing add-ons to create users directly would break this trust.
What is more likely to happen in the future is a “Request to create user” flow. Where an add-on, or non-admin, can hit a REST resource that puts the user in a backlog that the ‘site-admins’ could then approve. That would achieve both requirements, add-ons could start the process for getting users created while site-admins still have full control.
However, I can tell you that implementing this functionality across our products is, at this point in time, not going to happen in FY18. If you want to see something like that happen then I recommend that you raise an issue in the AC project on ecosystem.atlassian.net.
In the meantime, David’s suggestion is probably your best option.
P.S. You have not stated why you want to create users via your add-on. What are you trying to do?