If you are interested, Tau Beta Sigma's national website is:
http://www.kkytbs.org/
At the recent national convention there were over 800 attendees:
http://www.kkytbs.org/2003natconv.html
It's a small group compared to NPC et al groups but still of significant size none the less. We do all the things any NPC group does except participate in formal rush. At my school we would not have considered it. We do require you to be in marching band, but we had everything else, letter jersies, rituals, pledge periods etc etc.