It's something that I've noticed a LOT in this thread and it's been driving me nuts! And though I could probably get my head chewed off for this, I believe we, as Greek should know the difference [and I know there was a thread about this already]:

alumnus: singular, masculine
Joe Schmoe is an alumnus of XYZ Fraternity at ABC University.

alumni: plural; masculine, also used when describing a group of men and women
Joe and Bob are XYZ alumni.
Bob and Tina are alumni of XYZ university.

alumna: singular, feminine
Mary is a proud alumna of ABC Sorority.

alumnae: plural, feminine
Both mother and daughter are alumnae of ABC Sorority.

Thanks...I just needed to let that out.