Ok, you posted here in a thread about ispconfig 2, thats a completely different software then ispconfig 3. So what you find in this thread does not apply to your server.
Mailusers are grouped by domain name, there are no additional groups available. If you want to group users in a different way in ispconfig, you might have to extend ispconfig. If you want to do the grpouping in roundcube instaed, you might want to contact the maintainer of the roundcube ispconfig 3 plugin in this thread: