How to assign usergroup to users from PHP?

[Genent]

Hi,
I would like to do the following, but it's not exactly clear how (is there a better doc on Data Managers than on vbulletin.com?)

So basically we have a membership based paysite, and we are running a forum on a separate domain for our visitors.

Everyone can register on the forum for free, but we would like to give access to a few extra forums for the paysite members.

We have a completely separate user database for the forum and for the paysite.
On the paysite members will be able to give their forum username and password in their profile, and they should get VIP access to the special forums.

I made a usergroup called "VIP Member" on the forum, and I would like to automatically assign this usergroup to every user that provided his username/password on the paysite. (and remove them from this group when their paysite membership expires)

What I could figure out is that I should use data managers, but there is very little documentation on how to use usergroups for instance.