The best advice I can give you is to work on getting your GPA higher. This is something that will stay on your transcript FOR LIFE!! To be concerned about joining any sorority is not as important as improving your GPA.
I can't speak for other sororities but as for mine, you do not have to be in grad school to join a grad chapter. But you should have some accomplishments and be somebody who is able to contribute to the programs and service of the grad chapter. You also have to be extended an invitation to join. You will not be even invited to a formal rush or anything membership-related if you are not an accomplished, confident woman worthy of membership. So in other words, yes your undergrad years will still count so work on what you can do to finish off your senior year well!
Please concentrate on your studies and improving your GPA. You will have difficulty getting certain jobs, getting into grad school, etc. if your GPA is that low.
