They are always formal in my opinion...just like any professional job interview. When you first meet someone in this set of circumstances, you should even allow for being slightly more formal than projected in invitations sent your way.
For men, that means a suit and tie. Period. For women, that means the equivalent. I think a few possibilities such as a business suit with dressy flats or heels, a skirt/blouse, or a dress pants/blouse combination (with the same dressy flats or heels) would all be appropriate. I would keep makeup, hair, cut of clothing, and jewelry all more conservative than not.
I cannot believe I just attempted to answer a woman's fashion query. Oh, how the mighty have fallen...