facebook group marketing basics

plz help me in below
1) which group gets more member join? open or closed?
2) should only admin post or other also

Keeping Group "Open" or "Closed" will not make much difference. Getting more requests depend on how good your Niche is and how good name you keep.

Regarding Posting

You should keep Posting available for everyone, Since if you keep posting only for Admins then your group will not remain active.

content is just viral images,Indian funny jokes