It's my understanding that the sororities (and fraternities) were not invited by Harvard. They are not recognized by the College as student organizations; neither are the "final clubs" or the women's societies (equivalent to the men's final clubs). The rationale, I believe, is that Harvard will not recognize single-sex student organizations.
However, back in December 2004
The Crimson ran a story reporting that the Committee on College Life's subcommittee on Harvard Student Organizations was hoping to devise a new or updated recommended policy concerning fraternities, sororities, final clubs, and women's societies.
http://www.thecrimson.com/article.aspx?ref=504791
They were looking at spring 2005 as a possible time for the revision. Where that proposal may go is anyone's guess; right now it's my impression that the uproar over President Summers is taking precedence over anything else in the workings of the school's bureaucracies.
