What if someone tries to register at a second site with the login name of someone else that already exists the first site?
Then they get an error upon registration like the below:
If you register on a slave, it also gets registered to the master. If you register at the master, then you are registered at the master only, so... regardless where you register, everything calls back to the master to see if that username or email is in use or not, thus resulting in the above error for you to choose again / supply a unique email address.
Will that name be rejected unless they know the corresponding password ?
That name cannot be registered, period, on any SSO site.
If you had that name, then you just use it with your password on any site within your SSO network to login.
Whilst I cannot say every foreseeable option has been catered, I have tried to break the system every way near possible that I could think of, and I came up with some creative ways, and every time a way was found, Nathan has built in an error redundancy or solution to cater the issue.
Adding another good example, I register on a slave as xen01 with email address. That also registers at the master account.
Now I try and register a different username, xen02 at another slave, using the same email address as I used with xen01, and the same resulting error:
I will say, this has been thoroughly tested using default Xenforo registration fields.
Things may become more complicated with people using custom fields that are required, as SSO is not designed, nor could ever be possibly catered to authenticate every custom field option. How it will handle custom fields has not been tested, as they are not default Xenforo registration requirements.