Well, you made the statement "logged onto the domain" clear enough, but I don't think your understanding of what this means, given that you want to "reboot the server," is correct.
Although we often use the statement "logged onto the domain", the actual physical "log on" is to the local machine. But a user can "access" the domain via any connections to the server regarding open documents and active connections/sessions. This is because the machine and user have membership in the domain, and therefore have security token which the server uses to allow access.
In order to reboot the server, all you need to do is make sure no-one has any active connections to the server. The "domain" is a virtual thing used for organisation and security. You don't actually "log into" a domain. Though I'm sure someone out there will have a hair-splitting argument against this!
However, you should note that if you reboot a server, and there are active connections, they should be restored without problem.
The only main problem will be if someone tries to save a document during the reboot, and sometimes this can cause a lock-up of the app on their local machine, and when this happens, either the app will crash, or some people will get impatient, close the app manually, and possibly lose work.
As to your question of who is "logged into the domain?" At the command prompt, you can use the command "net view", and even stipulate specific domains if you have several, and this will show you which machines are currently on, and in the master browser list. This may be more what you want.
But I should reiterate, that when you reboot the server, more important isn't what machines are "on" and "on the domain", but which users have active connections to the server you are about to reboot. These are where problems are likely to occur.
hth,
Will
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