Add Guest account created during login to certain groups?

Solution 1:

The guest accounts are created by the shell script /usr/sbin/guest-account.

At the end of the function add_account it calls useradd to create the user. If you add the -G plugdev,dialout option here that should do the trick.

Use dpkg-divert to avoid that an updated package overwrites your modified script.

Solution 2:

Use usermod, not 'moduser'.

... so the behavior of the startup script prefs.sh now works as advertised. By creating a file called prefs.sh in /etc/guest-session with the following line:

moduser $USER -a -G plugdev,dialout

That command will cause the guest session to crash and not load if you include it in prefs.sh. '`moduser' isn't a valid command. Use this line instead:

usermod -a -G plugdev,dialout $USER