In addition to what gabi posted, try the FAQ 
(http://plan9.bell-labs.com/wiki/plan9/FAQ/index.html).

>- After creating a new user, the new user does not (yet) have a
>  password. Apparently, I cannot use auth/changeuser because I don't
>  have an auth server (the system is setup as a "terminal" whereas I
>  would normally consider it a "workstation") and passwd gives me
>  "protocol botch: cs: can't translate service".

You need to setup your ndb to recognize the auth server for your host, 
and you need to run one, as well (terminals, by default, don't. See the 
wiki page 'Drawterm to your Terminal').

>- How do I logout to let another user login?

As quanstro said, reboot.

>- Is there something like "su" to let me be someone else in another
>  window?

There's auth/login (auth(8)), or you can CPU to the same host.

>- How do I stop rio to get to textmode?

Again, slay rio|rc, but I don't know why you'd want to do it.

>- Can I somehow lock the screen?

You could run a shim before you start rio, or edit rio. There is a 
screen lock program in /n/sources/patch/sorry, and probably one or two 
in contrib.

>- I noticed that users glenda, adm and none don't have a password (by
>  default). These users can however change the timezone, reboot the
>  machine, ...

Whoever starts the terminal is the hostowner. He can do things like 
rebooting it. When you run an auth server, it performs authentication 
for filesystems, and, by proxy, CPU servers. Only the hostowner can 
reboot the system, for instance, so you can't just CPU to tip9ug and 
reboot it.

>- What is the purpose of the different users like (a) glenda (seems to
>  be hostowner, can create users, etc.), (b) adm, (c) none?

See above.

>  [*] is part of the sys group but could not change /rc/bin/termrc
>  because he couldn't write to /tmp (no profile like a normal user
>  binding /tmp to /home/tmp; I suppose he can do this interactively)

Running ramfs, for instance, would serve the purpose. Fortunately, Plan 
9 doesn't have the /tmp mess that Unix has. Users bind their own tmp 
directories over /tmp.

>- Is there something like virtual consoles to allow e.g.  several
>  users to login simultaneously and each starting a graphical
>  environment?

No. You could run multiple rios, or alter rio in some way to simulate 
it, easily enough.

>- Is there something like a plain "mount" command, just to see all
>  bindings?

cat /proc/$pid/ns

-- 
Kris Maglione