Data reduction in QlikView is completely data driven so you do not need to enter any conditions like OSUser()='IAN'.
Instead you should make sure you have a field in your Section Access that exists in your data also. Both field name and field values need to be in upper case. I assume that your field "GROUP" is such a reducing field, and if so you should just make sure that you have it in your data, too, with proper logical links to other fields.
Then you should check "Initial data reduction ..." on Document Properties -> Opening. And that's all.
What you mean by "either on the NT name or the group", I do not understand. NTNAME is only used to match the authenticated user (or AD group) and additional fields, like GROUP are only used for authorization, i.e. data reduction.