In the HashiCorp stack, where's the appropriate place to add users?
I am less familiar with what Azure offers, but other cloud providers offer a way to manage users external to the VMs the provide (normally IAM permissions).

I would not bake user into the system but look for an external way to manage them. This allows you to more easily remove users that leave your company or add new users that join.

If Azure doesn't provide a way I would look at setting up Active Directory or LDAP to manage user permissions to hosts. But I would be shocked if you needed to do this.

Note: Don't confuse infrastructure and data. Users are data, not infrastructure. Data (ie adding a user) should be able to change without changing the infrastructure. If you need to remove a user you do not want to rebuild your whole infrastructure.