Breathesgelatin suggested a thread listing information on all kinds of different sororities for chapters looking to affiliate with a national organization. So here is the start of it.
I've tried to make a list of all the national sororities I'm aware of. I may have left some out. (Not all of these are within my areas of expertise!) Please add any organizations that are not on here. And if you have expansion information, please post that as well.
At many schools, a new Greek group of any kind may require administrative approval. This thread assume you can do your own homework about your school rules.
National Panhellenic Council groups
http://www.npcwomen.org
(At schools with a Collegiate Panhellenic Council, the vote of that council is required to bring a new NPC group on campus. A group can be added two ways. One, the council can vote to expand, NPC groups apply, the council selects one, and that one colonizes from the ground up. Two, a local organization successfully petitions the council to invite an NPC group, that local invites the NPC of their choice to colonize them, and if the group agrees than the local becomes a colony. All NPC groups have websites.)
Pi Beta Phi 
Kappa Alpha Theta 
Kappa Kappa Gamma 
Alpha Phi 
Delta Gamma 
Gamma Phi Beta 
Alpha Chi Omega 
Delta Delta Delta 
Alpha Xi Delta 
Chi Omega 
Sigma Kappa 
Alpha Omicron Pi 
Zeta Tau Alpha 
Alpha Gamma Delta 
Alpha Delta Pi 
Delta Zeta 
Phi Mu 
Kappa Delta 
Sigma Sigma Sigma 
Alpha Sigma Tau 
Alpha Sigma Alpha 
Alpha Epsilon Phi 
Theta Phi Alpha 
Phi Sigma Sigma 
Delta Phi Epsilon 
Sigma Delta Tau 
NPHC sororities
http://www.nphchq.org/
Alpha Kappa Alpha - 
http://www.aka1908.com/aka/index.htm
Delta Sigma Theta - 
http://www.deltasigmatheta.org/
Zeta Phi Beta - 
http://www.zphib1920.org/
Sigma Gamma Rho - 
http://www.sgrho1922.org/
Non-NPC NPC-style groups
Kappa Beta Gamma - 
http://www.kappabetagamma.org/
	Expansion info: 
http://www.kappabetagamma.org/c-newchapter.html
Alpha Sigma Kappa (engineering) - 
http://www.alpha-sigma-kappa.org/
Phi Sigma Rho (engineering) - 
http://www.phisigmarho.org/
	Expansion info: 
http://www.phisigmarho.org/Expansion.htm
Ceres (agriculture)
Non-collegiate sororities with collegiate chapters
Beta Sigma Phi - 
http://www.betasigmaphi.org/index.shtm
Epsilon Sigma Alpha - 
http://www.esaintl.com/
	Expansion info: 
http://www.esaintl.com/collegiate/
Service sororities
Gamma Sigma Sigma - 
http://www.gammasigmasigma.org/
Omega Phi Alpha - 
http://www.omegaphialpha.org/
Professional Fraternity Association groups with an undergrad single-sex focus
Sigma Alpha (agriculture) - 
http://www.sigmaalpha.org/
Sigma Alpha Iota (music) - 
http://www.sai-national.org/
Multicultural sororities
Theta Nu Xi - 
http://www.thetanuxi.org/
	Expansion info: 
http://www.thetanuxi.org/expansion.html
Delta Xi Phi - 
http://www.geocities.com/~deltaxiphi/
	Expansion info: 
http://www.geocities.com/~deltaxiphi/fall02main.html
Zeta Sigma Chi - 
http://www.zetasigmachi.com/
	Expansion info: 
http://www.zetasigmachi.com/expansion.htm
NALFO sororities
http://www.nalfo.org/
Alpha Pi Sigma Sorority
Alpha Rho Lambda Sorority, Inc.
Gamma Alpha Omega Sorority, Inc.
Gamma Phi Omega International Sorority, Inc.
Kappa Delta Chi Sorority, Inc.
Lambda Theta Alpha Latin Sorority, Inc.
Lambda Theta Nu Sorority, Inc.
Lambda Pi Upsilon Sorority, Latinas Poderosas Unidas, Inc.
Latinas Promoviendo Comunidad/Lambda Pi Chi Sorority, Inc.
Hermandad de Sigma Iota Alpha, Inc.
Sigma Lambda Gamma Sorority, Inc.
Sigma Lambda Upsilon/Seņoritas Latinas Unidas Sorority, Inc.
Corazones Unidos Siempre
Chi Upsilon Sigma National Latin Sorority, Inc.
Omega Phi Beta Sorority, Inc.
Asian interest national sororities
Alpha Kappa Delta Phi - 
http://www.akdphi.org/
	Expansion info: 
http://www.akdphi.org/kdpnew/index2.htm