I don't see that being called by the user panel. See if adding the user on the Settings -> Access Control Lists -> User Groups tab works. I doubt if it will work.
I think that event is for future use. I could be wrong.
I think adding this code at the end of the modUser class joinGroup function would do it (just above the 'return $joined;' line around line 701):
if ($joined) {
/* invoke OnUserAddToGroup event */
$modx->invokeEvent('OnUserAddToGroup',array(
'user' => &$this,
'usergroup' => &$userGroup,
'membership' => &$member,
));
}
Of course it's very much *not* recommended to hack the core code and it would be overwritten when you upgrade MODX.
A safer solution would be a plugin attached to OnBeforeUserFormSave and OnUserFormSave that checked to see if the user was added to any new groups, and performed your operations in that case. It would be very tricky, though.
I think you'd have to get the user's groups in OnBeforeUserFormSave, save them to a $_SESSION variable, then get them again in OnUserFormSave and compare them to the ones in the $_SESSION variable, then unset the $_SESSION variable. I'm not sure it would work.
