Alrighty, as far as I know, the Nu Phi Mu chapters were for starter chapters. I believe that there used to be a rule where you could only join a chapter of which you belonged to that degree. Because you can join as a Ritual of the Jewel member at the age of 22, being of the Nu Phi Mu degree is often overlooked. Our chapter happens to be on a college campus, but by no means do i think that it has to be a Nu Phi Mu or on a campus. According to The Book of Beta Sigma Phi, the Nu Phi Mu Degree is the first degree of Beta Sigma Phi, and Invitation to Life merely says that Nu Phi Mu is where most members ages 18 to 22 would join.
I know that I am a little worried about moving out to seattle after my college graduation because I am only of the Nu phi mu degree, will be 22, and won't be eligible for progression until after April.
I hope that helps, and is right!