I know it\'s possible to get an rss feed or whatever for wall posts of groups. However, every time I try it with a closed group that I belong to, it doesn\'t work. Is this possi
To answer your question, yes you can access the posts even if you are not admin/owner. If you can see all posts of a group, there should be no reason not to be able to access them with the api.
First, check if the access token is having 'user_groups' checked, otherwise you will not be able to access a closed groups. You can play with Graph Explorer yourself, till you get the needed results.
If you experience problems after you get the right access token, then check if the token is not expired.
You should switch to v2.3 because from version 2.4 facebook restricted with closed groups and also user's new feed.
Try with v2.3 and check on read_stream on get token access. Note that only by switching to v2.3, we can see the read-stream option
https://tngotran.wordpress.com/2017/02/01/facebook-api-explain-example-and-real-project-tutorial/