Only Arts & Sciences Computing staff can add new users. However, if you are a group administrator, you can add any existing user as a member of your group and give them a certain set of permissions (i.e. a role).
By default, the first 250 characters of the post (page, article, etc.) will display on lists described as "(summaries/teasers)", on the home page, and as blocks in the sidebar. There are a few ways to customize this summary.