Management of the announcements that appear in the home page are by default reserved to the domain administrator (admin). The administrator can allow other users to post/manage announcements.
The announcements that appear in the home page are taken from a message board chosen by the administrator. It is possible to select what message board is used to display announcements (by default there is a specific message board, called announcements), how many of the latest announcements to display, and whether to display announcements at all or not.
These preferences are set in the Administration / Announcements section (available only to the Administrator).
The default Announcements board can be written only by the admin user. To allow other users to post announcements, simply create a new share with write permissions for the board, and give access to the selected user(s).