Allows your user account to 'su' to root. Recommended on your primary user account for easy maintenance. Also used with sudo.
audio
Allows your user account to directly access audio devices. Required if using ALSA; otherwise optional.
plugdev
Allows your user account work with various removable devices. Allows adding of a WiFi network in GNOME without providing root password. Recommended for desktop users.
portage
Allows extended use of Portage as regular user. Recommended.