You can make changes to the setting in Admin Panel> Under "App Settings"> Click on Groups> Turn off the option that says "Users can post to the homepage"> Save.
This way, when the user wants to post something from the homepage input box, the system will ask them to pick a "group" as a mandatory task. This way, all the posts in the community will be shared in groups.
For the groups that you have enabled the group notification9 for, all group members will receive a notification when there is a new post in that group. It might be overwhelming for the other community members.
Therefore, if your concern is to know when there is a new post in the community, you can check out our suggestion here21.