I found out that nowadays many DE lack an application to manage groups. Usually it is user settings, but there is no way to add or remove groups, join a group and so on. I found out that Ubuntu MATE actually has it. See attached screenshot.
I know it might sound not that big of a deal to do it from terminal, and that’s how I have been doing it for a long time. But I believe it would be quite handy for new Linux users or people that just want to get into Linux but not touch the terminal.