Hi
The About tab will only show once you put content into it, assuming you have the default config set this way (hide empty tabs).
If you are logged in as the Group Owner or Admin you have access to About in the settings dropdown, which appears bottom right of the canvas. Just select about and you can enter stuff there.
You can also find it under the Integrations tab if you edit the group in the back end.
make sure that you have also published the GJ About plugin as well of course. Groupjive which is a plugin itself, has a number of plugins, including About.