Err, in another thread it's said that users should belong to the "audio" group if they want access to the sound system (i.e. otherwise error dialogs only go 'ding' for the user who was the first to log in).

Are there other groups that one should be member of? This is for a family computer, there should basically be "root" and then "everyone else" (so the whole family gets the same physical access, but retains individual desktops, settings and stuff).

" This is for a family computer, there should basically be "root" and then "everyone else""
This is not a good idea to be root in normal circunsptances. It's better to enter the password for root if needed.

Look around for groups in the configuration centre on your desktop (you'll need root priviledges, but you can access through a normal user session. Read through the availabe groups and you'll be able to recognise a couple like cdrom, tty (should be diskette) and maybe others for internet access (names can vary - www, ftp etc.)
And as apimente.br said, it's safer to leave the root account for admin tasks only.

@apimente.br:
Yeah I know I shouldn't be running as root, and I'm not. I just want all the "regular" users to have the rights they (we) actually need.

@linmix:
So we should all belong to the groups "cdrom", "tty" (and that's the floppy?), "www", etc? Wow, I had no idea. Either I have misunderstood you, or every Linux guide I ever read has a serious omission. Please, if you will, elaborate.

I'm no expert on this matter, but in order to have access to the resources of the system you need permissions. In order to make linuz a secure system (or rather a controllable system) no need to specify which users will have access to which resources. Depending on the settings of the permissions you may be able to only read a file or also write to it. This is achieved by the permissions of the files which you can see when you type

$ ls -l

You'll see something like:

-rwxr-x--- user user

at thebeginning of a normal file.
The first symbol (-) indicates this is a file (not a folder: d)
The rest of the digits are read in groups of three and indicate permissions for the owner of the file (rwx), the group the file belongs to (r-x) and all other users (---). The first 'user' you see is the name of the owner of the file, the second 'user' is the name of the group. This way you can make files available to members (and e.g. avoid them changing the file) of a group but not to any other user. By making a user member of a group (like 'user') you give him the possibility to share the file to the extent of the permissions set for the group.
If you don't want a user to be able to use the floppy drive then you don't include him in the corresponding group and avoid his taking things out of the system or adding external files.