Control user groups based on a remote lookup

Hello all,

I'm looking to create something that can pull a users profile information, checks a 3rd party site based on some of the account and profile information, then modifies their group depending on what is returned from the 3rd party lookup.

I have most of this covered however I'm at a loss on how to actually pull all users then pull a specific users information, then at the end change their group? Is there like a current example or a how-to for 4.0? I found a lot of out of date information while looking.

Any help would be greatly appreciated. Thanks,
-Rob