How do I access the properties of an object which triggered an event?
Within the context of that event, you will have access to:

$event-&gt;params['user']


... which is:


  A UserModel instance representing the user that has just been activated.


Once you've got your user model, you can do something like this:

$event-&gt;params['user']-&gt;isInGroup('artists')




On a separate note, I believe the correct syntax is:

craft()-&gt;on('users.activateUser' ...


You can ditch the "on" in onActivateUser. Check out the official example...