There is a pretty recent thread on here that outlines many of the pros and cons of participating in a colonization.
http://www.greekchat.com/gcforums/sh...d.php?t=132202 Use the search function for more info, or use Google "Greekchat sorority colonization" or similar.
Regarding social stigmas and joining a new sorority--it will depend on your campus culture, but that shouldn't play into your decision and as an adult (you're going to be a senior), hopefully you can see beyond that. What's important here is creating sisterhood, and if you're happy, it doesn't matter what other people think. Please don't pay attention to ranking sites (seriously, block yourself from visiting)--they're all nasty BS.
I wish you all the best with colony recruitment and hope it works out for you. It's worth giving a chance. Though there are no guarantees, generally a colonizing sorority takes women from across all class levels including seniors.