cb helloworld plugin for actual cb 1.7. and joomla

Ok,
I want to try a possibility to make a GroupMessage OnAfterWallCreate.
I want to try to send a email to all users of a group, which says in which group and category is a new wall entry.

(you said, perhaps you will some day make such thing, but it's not on your priority list. For me it's important, so I have to try for myself!)

In that case you'll need to review the trigger tutorial and the example as provided from the tutorials link in my signature as it explains in detail how to use CB triggers. What you're trying to create is a trigger based plugin. Please review GJ integrations as well to understand how to use GJ triggers. You may want to also find that trigger in GJ wall integration (search its source) to see what variables it provides so you know how to setup your function. GJ provides all the API necessary to obtain the users as well as send the email, please review source closely.