Cannot modify group members of existing group containing underscore.

[Kyle Horodyski]

I have 2 user accounts on my system:
  1: user
  2: user_name

I want to add them to each other's groups so that by default they can always modify each others files.

When trying to modify group "user_name"'s properties in the users admin application, clicking OK gives me a popup window with the message:

"Group name has invalid characters
Please set a valid group name consisting of a lower case letter followed by lower case letters and numbers.".

As the group name already exists and can't be changed from its properties window this seems like a contradiction.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: gnome-system-tools 3.0.0-4ubuntu2
ProcVersionSignature: Ubuntu 4.4.0-116.140-lowlatency 4.4.98
Uname: Linux 4.4.0-116-lowlatency x86_64
ApportVersion: 2.20.1-0ubuntu2.15
Architecture: amd64
CurrentDesktop: LXDE
Date: Tue Apr 3 14:58:25 2018
EcryptfsInUse: Yes
ExecutablePath: /usr/bin/users-admin
InstallationDate: Installed on 2018-03-04 (30 days ago)
InstallationMedia: Lubuntu 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1)
ProcEnviron:
 LANGUAGE=en_US
 PATH=(custom, user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: gnome-system-tools
UpgradeStatus: No upgrade log present (probably fresh install)