Where are you creating this formula? You should be in view->field explorer->formula->new. Also, what error message are you getting? You don't really need the parens, but that shouldn't cause the formula to fail:
If {Organization.Work Status} In ["HrlyFT","HrlyPT"] then "H"
Else "S"
Also make sure the syntax is set to "Crystal", not "Basic".
-LB