member, person

員 is composed of 口(mouth) and 貝. 貝 in here is actually the variant of 鼎(cauldron), the mouth of most of the cauldrons is a circle, so the original meaning of 員 is circle, but such meaning has been replaced by 圓. One cauldron has only one mouth, so 員 can also be used as the unit of quantity. In addition, cauldron was seen as an important utensil in ancient China, while a man is also an important constituent of any organization, so 員 is also used to mean person or member.