shin-gao, "a new face".
Many students told me that one fascinating thing about an intensive study abroad experience is to meet many new faces day after a day. It certainly due to the hospitality of the host organizer and kind young local students. There, this word came to my mind. 新顔 means "a new face". However it just can not be used in a situation described above. An underline understanding with this word is to refer a new one who is expected to grow into a regular member, but not some one who is new and may well disappear easily. Further more, this word is often used in a metaphorical manner, such as to refer a new type of product.
Thus the opposite word here is not "old-face", rather it is 常連 (jou-ren) which means a regular member.