Something you ought to know is, from the manual page for gpasswd
Group passwords are an inherent security problem since more than one person is permitted to know the password. However, groups are a useful tool for permitting co-operation between different users.
Basically, you don't want to fiddle with group passwords 'cause you can create a mess without really trying.
The "better" way to add a group to a given user account is the usermod
utility (and you need to be careful with it, too:
usermod -a -G group[,group,group,group] userid
is the proper syntax to use to append
group(s) to a userid.
There really isn't a good reason to use group passwords in virtually all normal operations; for what it's worth, I've never, in over 30 years of working with Unix/Linux systems, needed to use group passwords (been there, did that, didn't like it, undid it and stopped doing again).
In a normal (whatever normal may be) Slackware installation a user would need to be in these groups:
users lp floppy dialout audio video cdrom plugdev power usbfs netdev scanner vboxusers cvs
In the above, scanner
are added for (1) an all-in-one printer, (2) VirtualBox and (3) CVS. Other than those, this list should be complete. There would be added with
usermod -a -G scanner,vboxusers,cvs userid
Hope this helps some.