You can do this with an FQL query, for example:
SELECT uid FROM group_member WHERE gid = <group_id> limit 500
SELECT uid FROM group_member WHERE gid = <group_id> limit 500 offset 500
SELECT uid FROM group_member WHERE gid = <group_id> limit 500 offset 1000
...
( 0 ),
perPage = 500
for count in range(100):
res = fql('SELECT uid FROM group_member WHERE gid = %s limit %d offset %d' % (fbUserId, perPage, perPage * count))
if len(res) == 0:
break
friends += len(res)
FQL, :
SELECT uid, name, pic_square FROM user WHERE uid IN (
SELECT uid FROM group_member WHERE gid = <group_id> limit 500 offset %d )