[Sciencewoman]

While membership is for a lifetime, collegiate chapters look to sustain their membership numbers by generally recruiting younger members who will be involved for a few years -- forming friendships, paying dues, serving in offices, participating in social events, and becoming active brothers. I think this is something you should have pursued earlier if you "always wanted to join a social one." You can always try again, but your senior status is going to be a significant obstacle, because you're doing this at the last minute and you'll be gone next year.