they're referring to your bitcointalk member rank. you can't enroll while you're a newbie.
you should know this if you read their signature campaign thread.
I have no idea about the second question.
thanx.can you explain when my rank will be changed?
you will become Jr. Member when you reach 30 activity. you can increase your activity by posting, but it will only increase by max of 14 for every 2-weeks periods you posted in.
if you have more question about activity (not about yobit), ask it on that thread I linked.