Is it possible to get the member count of closed Facebook groups?
This isn't possible you can only retrieve 500 max currently via Graph API (https://graph.facebook.com/GROUP_ID/members)
All of the users who are members of this group (can only currently return the first 500 members)
Via FQL you can retrieve up to 5000 I believe
select gid, uid from group_member where gid = GROUP_ID
https://developers.facebook.com/docs/reference/api/group/
https://developers.facebook.com/docs/reference/fql/group_member/