I would advise you to ignore the email.
If anyone actually asks you about it, I'd look faintly surprised and say, "Oh, I didn't realize that included new hires! Since I haven't met (big boss)/haven't gotten on the payroll cycle/am not familiar with the protocol around here yet, I just assumed it was for the more senior staff members." Then remember that you don't have any cash with you, smile, and say, "I'll have to catch up with Secretary later."
Pressuring you like this is unbelievably inappropriate--especially if, as I suspect, you haven't even gotten your first paycheck yet! (In my experience, I never got a check on the first pay cycle, and pay cycles were 2 weeks each.)