All but the Pi Beta Phi house were built many, many, many years ago to hold anywhere from 55 to 100 girls. A few have been expanded in recent years, but none were ever meant to house
every chapter member.
Freshmen are required to live on campus, but I don't know if sorority housing is included in that. It might be, since UA housing is in a crunch right now. Generally, mostly sophomores and officers required to live in fill the houses.
The house Alpha Phi is currently occupying belongs to Sigma Delta Tau and is one of the smallest sorority houses (and always has been). Since their colonization was well past the first day of classes, they probably don't have anyone except the resident colony assistants (who are probably in grad school) living there right now.
I'd love to hear from an Alpha Phi about how things are going!
