Your campus climate is going to dictate whether or not it is possible/realistic to think you might get a bid.
That being said...you aren't going to be a typical PNM or a typical member, but you won't be the first ever. I had a friend who joined a sorority in her senior year. She had been a very close friend of the sorority since her freshman year. Finally, she decided to make her bond official, and she had a wonderful time. I haven't been in touch with her much, but I think she is still involved with her organization.
(I went to a tiny school where everyone knew everyone, and the whole campus seemed to be shocked by this girl joining! But who cares! She had fun.
)