There was a centralized computer which was programmed via punch cards which you could dial into to submit your specific simulation and later return to for results.
However, if you compare setting up a Plan 9 auth server to setting up a Kerberos server... well, basically anything to do with Kerberos makes me long for death. The Plan 9 auth system is one of the best things they made and I highly recommend checking out the paper: https://css.csail.mit.edu/6.858/2013/readings/plan9auth.pdf
But what machines are, in practice, multiuser today in that way? My work computer has only my account, and sometimes a temporary one if a technician needs to log in to troubleshoot something. For home use I don't think it makes sense. And for a prod network, again users aren't logging into the machines directly.
When this scheme was designed, it was frankly a little bit nutty. The CPU, auth, and file servers would be VAX or Sun systems, costing tens of thousands of dollars, and the terminals would be either slightly cheaper Suns or IBM PC-compatibles costing thousands of dollars themselves. Today, you could probably cobble together a passable network for a few hundred dollars, assuming you use cheap Atom boards for everything except the CPU server (which is meant to be a beefy compute box, but let's be honest nothing in Plan 9 uses a lot of cycles). This makes the architecture more sensible than ever.