I'm a grad student. I was recently initiated into Phi Sigma Sigma Iota Gamma Chapter (we rock!!!)
Phi Sig is one of the only NPC organizations that initiates graduate students into its collegiate chapters.
It's a really great experience. As you said, some of us were unable to experience Greek life as undergrads for various reasons and our lives are sometimes a little more "complicated" for lack of a better word.
Not only am a graduate student, I am a returning student (I'm the old lady of the group), I have a full time job and a husband.
Phi Sig is important to me so I make the time. There are many days when I'm so exhausted that I can hardly see straight. But the sorority picks me up and I'm ready to go.
My sisters are really wonderful.
I encourage anyone who wants to be part of Greek Life regardless of age or education level to just do it. You will not regret it.
As a graduate student I think I set an example for my younger undergrad students. It shows them that they can still be part of the sorority and be active.
I hope this helps. Go for it. Start that group. If there are Phi Sigs at your school, check us out.