This class would belong more appropriately within the 'user' API
(core_user) instead of within the 'core' API, since it is
directly related to user data.
Since the class has only just been added to Moodle, now is a good
time to move it.
In all cases changes have been kept to a minimum while not making
the code completely horrible. For example, there are many instances
where it would probably be better to rewrite a query entirely, but
I have not done that (in order to reduce the risk of changes).
Mainly to verify groups visibility this new callback has been created.
Note this was originally 3 commits but for amending purposes they have
been squashed.
Andrew Nicols