Social Feed, Crowd Pics and Announcements are the same kind of page with different settings, so they all work the same way. What you can do on one depends on what the app's creator chose: on some pages everyone can post, on others only the organisers can.
Posting
Posting to a social feed or Crowd Pics page — text, photos and videos, from your phone or tablet
Posting from the iOS share sheet — sharing a photo to Yapp from another app
Signing in to post — why you are asked for an email address
Your own posts and comments
Other people's posts
Notifications
If you are the one who built the app
There is a separate guide to setting these pages up: Getting Started with Social Feed Pages Guide.
