vB_DataManager_User for Preexisting Users?

[Irwin]

I wish to make a quick authentication check script outside of vBulletin which utilizes the vB_DataManager_User class to verify that a given user is a member of a certain user-group.

So far I've seen plenty of examples for vB_DataManager_User, but they're all for making new users or relying on preexisting logins so my question is; how does one access a preexisting account using vB_DataManager_User? I wish to do something like this:

check.php?user=<user>&pass=<pass>

From there on which the script will verify the user-name & password then see if the user is part of the correct user-group.

How would I go by doing this?

Thanks in advance.