Originally Posted by sycamorex
Try [as root]:
usermod -a -G audio,video,cdrom,plugdev,power,netdev your_username
Slackware 13's usermod does not appear to have the "-a" option which I assume showed up in -current and which I further assume means to add you to each group in the list versus setting your groups to that list.
The following command will add you to the these groups in addition to whatever groups you already have configured and will work on older versions of Slackware.
for x in audio video cdrom plugdev power netdev ; do gpasswd -a your_username $x; done