SQL Help

Anyone want to try to help me with this. I have a DB with 2 tables (users and instance) a bunch of records were deleted from the users table, and I want to find record id numbers and a record name from the instance table that have userid's that have been deleted and that are of a type "Catagory".... I've got most of the statement working, but the "userid not in users.userid" doesn't because I'm not sure how to refer to the column as a list....

SELECT t1.userid, t1.iid, t1.name FROM instance t1, users t2 WHERE ( ??? ) AND ( t1.IsA = "Category" ) ORDER BY t1.userid ASC;