You’ll need to be invited to join a group chat. If you’re invited, you’ll see a prompt within your 1:1 chat with a friend. If you hit ‘Join chat’, you’ll enter the group chat as long as the maximum occupancy of 15 hasn't been reached. Other members in the group chat will also be able to see that you’ve joined.
If you don’t want to join the group chat, no worries. You don’t need to do anything.