"Require group" doesn't work with Ubuntu's default Apache installation

If you want to use Require group $GROUPNAME on your default Apache installation like this:

<Directory "/var/www/foobar">
    Order allow,deny
    Allow from all
    Options None
    AllowOverride all
    AuthName "Area 51"
    AuthType Basic
    AuthBasicProvider external
    AuthExternal pwauth
    Require group admin
</Directory>

You need to first install libapache2-mod-auth-sys-group. You don't get any error message if you haven't installed it but it doesn't work.

sudo apt-get install libapache2-mod-auth-sys-group