Here are some resources for learning Gregorian Chant and the traditional Latin Mass.
The following site provides recordings and scores for the Ordinaries and Propers for the entire ecclesiastical year! By playing the recordings daily, your group can slowly and deliberately learn the chants of the Church.
If learning the Gregorian Chant propers is daunting for your group, try learning the ordinaries for the Missa de Angelis and Orbis Factor Masses. Then use the simplified Rossini Propers.
Guide to Square Notes
A Gregorian Chant Master Class
The Chant Cafe
This is one of the best hymnals available.
Getting Acquainted with the Latin Mass
This is an excellent interview of Father James Fryar, FSSP, who explains the traditional Latin Mass.