I need to promote FB group.
Is it effective to use paid FB traffic?
What are average CTRs of people joining the group from paid ads?
Yes it’s effective to use FB ads to promote FB groups. You can get likes for as cheap as a few cents if you do it right. I would recommend starting with very small tightly targeted campaigns related to your group. Once you see which ad copy and demo works best then slowly go broader and broader.
Can’t really be to specific with advice since you have left a lot of info out.