It’s complicated. Official Facebook API rules are:
Regular Facebook API allowed to post:
1. Facebook Profile.
2. Facebook Pages (all kinds) (User who authorized the plugin must be an admin of the page).
3. Public Groups (User who authorized the plugin must be at least a member of the public group).
4. Closed groups (User who authorized the plugin must be an admin of the closed group).
5. Secret groups (User who authorized the plugin must be an admin of the secret group).
However, there is an unofficial workaround. Facebook issued several special apps with extended permissions that can post to closed/secret groups if user who authorized it is just a member. You can authorize such app and get an auth token. This token will allow you to post to any closed/secret group where you just a member. You can enter this third party token in the “Advanced Tab”
How to get a token and configure it.
1. Find the app and it’s ID. (Please do not ask us where to find the app. We are not allowed to share that info.)
2. Visit this URL (replace XXXXXXXXXXXXXXXXXXXX with actual App ID):
3. You will get the standard Facebook authorization dialog, answer “yes” to everything. You end up on the blank page.
4. Visit this URL (replace XXXXXXXXXXXXXXXXXXXX with actual App ID): https://developers.facebook.com/tools/debug/accesstoken/?app_id=XXXXXXXXXXXXXXXXXXXX
5. Add new “Facebook” account to SNAP, enter anything to key and secret, click “Add Account”, open it’s settings, copy-paste that token to Advanced tab -> Auth token.
6. Enter numeric groups ID to the “Where to post” field.