how do they decide who gets to live in the

This varies from organization to organization. Some chapters do a lottery (ex Kappa- but they had an issue with certain friend groups coincidentally chosen and rumor that it was not a true lottery). Other sororities use a point system where girls can contribute to cleaning or helping with fundraising events on campus to earn points and those with the most points will then earn a spot on the sorority halls. Many girls also opt to live in suites with friends in other sororities. Additionally, the president and maybe a few other leading officers may be required to live in the sorority housing block, depending what their nationals require.