This applies to Social Feed, Announcements and Crowd Pics pages — the same page type with different settings.
What gets sent by default
People who have your app get a notification when:
a new post is added to the page
somebody likes or comments on a post they wrote
What you control
For each social feed page, you choose whether it notifies by default when somebody installs your app. Click the gear icon next to the page in the Editor's page list. Publish your app afterwards.
This is the setting that makes an Announcements page work the way people expect: every post goes out as a notification, which is the point of the page.
If a page is for conversation rather than announcements, and you expect a lot of posts, consider turning default notifications off so your attendees are not interrupted constantly.
What your attendees control
After installing, each person decides what they receive. They can:
turn notifications off for a particular page
stop being notified about likes on their posts while still hearing about comments
follow or unfollow any individual post
You cannot override those choices, and they can always turn notifications back on. The attendee-facing instructions are in changing your push notification settings.
How much of a post shows in the notification
A lock screen only shows the first part of a post, so the opening sentence is doing most of the work. Read How much of an announcement shows in a notification.
Sending a one-off message
If you want to reach everyone without it being tied to a post, see sending a message, reminder or push notification to all of your guests.
Reference
Scheduled posts and announcements — prepare a post now, have it go out later