@Faulkner1897 @donnaleighg Here are the official numbers for the 2015-2016 academic year from the Dartmouth Fact Book.
Sophomores 711 out of 1125 (63%)
Juniors 708 out of 1078 (65%)
Seniors 708 out of 1031 (68%)
So, if you are looking at eligible students, the numbers are 2127 out of 3234 (66%).
That year, the incoming class (freshman) was ~1110, so the overall % of students in Greek organizations is ~49%.