Recommendation Letters
 

Are people from schools that you are not attending allowed to send recommendation letters on your behalf to the school you are attending?

or in other words, is Susy able to send a recommendation letter to University of XYZ for ABC sorority, even though she was in ABC at a different university?

It depends on whether Susy is an active or an alumna. If she's an alumna, then the answer is yes. If she is an active, it depends on ABC sorority's policies.

Some schools may take Susy's recommendation more seriously if she attended University of XYZ or a school with the same recruitment culture, but that is overall less common.