to hang out: Often used by younger people - it means to be together with other people in a relaxed way. It involves doing as little as possible.

chilled: relaxed. "To chill out" means to relax.

the more the merrier: the more people there are, the more fun a gathering is. Often said when uninvited guests come round.

A:"What are you doing this evening?"
B:"Nothing much, just hanging out at home. Why don't you come over and watch a movie with us?"
A:"That sounds very chilled. When should I come over?"
B:"Whenever you want."
A:"Oh, I just remembered. I was meant to be meeting Bob this evening. Can he come over too?"
B:"Yeah sure, why not. The more the merrier."