You can publish to the Facebook group if you are using the premium or premium plus plugins.
Also, please more that you can publish to the Facebook group only if you are the admin of the group.
For this first, you need to add the Facebook app (your own app or the smapsolutions app) to your Facebook group. You can see the details in How can I add or remove an app from a group that I manage?
After adding the app to the group, please reauthorize the app in the plugin and you can select the Facebook group.
Note: Please note that to display the facebook groups after authorizing the app, they should be selected during the authorization process. Otherwise, you can go to the Facebook settings business integration tools https://www.facebook.com/settings?tab=business_tools, edit the app and select the groups permission, or delete the app and reauthorize from the plugin.