This is a fantastic question! Thanks for reaching out here!
Unfortunately, we do not have a way for RStudio Server Pro / RStudio Workbench to automatically provision users today 
As a result, there are a handful of usual patterns:
- provision manually (icky)
- use
sssd to connect to a directory / LDAP server that has the users in it and provisions them on the server
- create a manual script / cron job that queries the user store (i.e. Azure's API) and provisions users when new users are added
sssd / an LDAP domain join is probably the most common solution, but only works if you happen to have such a directory lying around. It is also worth noting that it is important to ensure that UIDs for new users are created in a consistent / idempotent fashion, and this is particularly important in Highly Available architectures.
Do you have thoughts on which approach seems most reasonable in your environment? I will share this feedback with our dev team - I am hopeful that at some point we will be able to provision users automatically, but that day is still unfortunately a ways out.