ECMC Group Inc., a student-loan guarantee agency in Minnesota, acknowledged on Friday a data breach in which the personal information of 3.3 million borrowers, including their Social Security numbers, was compromised. Guarantee agencies such as ECMC are the private entities that, under the U.S. government's system of federally subsidized student loans, collect government money and then turn around and pay it to private loan companies when borrowers default on their student loans.