You are one of the registered users. If the user has not registered with Teacher Focus or is a registered user who has not signed in yet, then he/she is a "guest" here, and "guests" cannot post.
In your profile, you have "Hide your online status" box checked - that's why you are "invisible" here even after you sign in. If you wish your username to be listed at the bottom of the page, simply go to your Profile and uncheck this box.
Hope this helps.