[2023] How do I join a countdown group?

I went to the chopper & selected countdown & was invited to a group. I have always played single so I do not know the details on how to join a group. Am I suppose to check join the group? I chose hard difficulty because I am new to countdown & no exp playing with a group.

Lastly I decided to do this because trying to up exp on a rifle it took the components & said not enough exp. What kind of hustle is is that? I had enough rifle parts before I started.