sororities that spring rush last year

Like previously said, a sorority will only do spring rush if their quota is low. So if two girls they gave bids to in the fall drop then they will probably have to add more. They probably aren't making the list now, but if you want to get a head start you will need to contact their Recruitment chairs. (you can find the contact info on the websites)

Tri Sigma will almost definitely do spring rush. Kappa will most likely not, they did really well this year. If Kappa does do spring rush then it will just be COBs. Pi Phi I would be surprised if they didn't have to do spring rush because they did not do very well in terms of numbers in the fall. Phi mu is in the same position at Kappa.
If you want to spring rush, you need to know the girls in the sorority. Last year how it worked was that Panhellenic had a kickoff in which all the sororities were supposed to be represented (even if they were not intending to bring in more girls). Then typically you have to be invited to rush a particular house by the sisters in the house. It is not very difficult to be invited, you just have to know a girl in the house and tell her that you are interested. If you rushed in the fall and received a bid, you cannot rush in the spring. But if you dropped out of rush before bid day or did not get a bid then you can still rush.