Hello Sunsail, Please Seat is wrong. It should be Please Sit Down. Please Be Seated is correct and in the office you can use it . Its a formal way to say Sit Down .
Please take a seat - also an invitation, but a bit more firm. (You don't really want the person to remain standing, but you're not ordering him to sit in a rude way.)
Please (pause - wave toward the chair) sit -- This sounds like a command but you're really saying "feel free to sit." Informal.