[TYPO3] Adding a user to a specific usergroup using PHP

[Esben Maaløe]

Hi List!

I am using the sr_feuser_register extension to allow people to
register themselves-

This extension comes with hooks already setup (nice!)

I've extended the feusers table with a property, and based on the
users choice when registering (or later on if changing this property),
I want to make the user a member of a specific group (and delete him
from the old group if relevant)

I have no clue as where to start with this (except of course that my
code should go into the appropriate hook), but my bet is that there
must be some Typo3 object that I can instantiate, which will provide a
method for me to use.

Any tips on where to look, and what to look for?

Regards
Esben