To be able to generate content feeds of Facebook groups and make posts to your Facebook group, two conditions are necessary: you should be a member of the group and RSS Ground app should be added to this group. You can make posts even not being a group admin. However, only group admins can add or remove RSS Ground app.
To add RSS Ground app to your Facebook group:
- From your News Feed, click Groups in the left menu and select your group.
- Click Settings in the left menu.
- Scroll down and click Edit next to Apps, then click Add Apps.
- Select RSS Ground app and click Add.
And you can see the list of your Facebook groups in the Facebook Feeds generator and Facebook Poster.