If you forget to add your user to a group, you can use two different commands with different options
To add the user to one random group
Code:
gpasswd -a <user> <group>
# OR
usermod -a -G <group1>,<group2> <user>
To reset all the groups that the user is a member of
Code:
usermod -G <group1>,<group2>,<group3> <user>
The default recommended groups for Slackware are:
floppy audio video cdrom plugdev power netdev
You may need to add or remove some of these or other groups based on your needs.