good at socializing; sociable; a good mixer
Describes a person who is skilled at building and maintaining social relationships, often in a light, friendly way.
She is good at socializing and quickly becomes friends with anyone.
People who are good mixers are popular at parties.
社交的 is a more common and neutral adjective for 'sociable', while 交際上手 emphasizes skill in socializing.
Compound of 交際 (socializing) and 上手 (skillful).