@benmore1992 Thanks a lot! you saved my life, because I've been preparing coding interviews. wow.

Wow, it's really painful, everything is not helpful for me. :( maybe I need to re-install Xcode.

Self Answer: I added GroupSessionMessenger on the Group Session for the activity. And looks fine so far. But looks like GroupSessionMessenger has some limitation such as data length or so. So need to send/receive compact data for GroupSessionMessenger.