With Postcron you can manage both Facebook Pages and Facebook Groups. For Pages, you can direct-post your content, and for Groups you can post via notification. You can find more information about posting with notifications here.
For Facebook Groups, you don’t necessarily need to have any admin role or authorize your post, you can publish by simply being a member of the Group. Here you will find a complete tutorial on how to connect your Groups. It's a very simple process!
Once your Groups is connected to Postcron, you’ll be able to easily schedule your content. When it's time to publish, you will receive a notification on your mobile device to finish posting.