My fraternity is classified as a professional GLO. On my campus we also had an APO chapter. I recall more than one mixer between our two chapters. They weren't treated exactly like "real" greeks (meaning the social GLOs), but they were considered part of the greek system. They were on the Greek Council as well. (I think this was okay because there were NO NPC or IFC chapters, and because there were 2 PFA chapters).
And as far as that particular chapter went, they held similar functions as the other greek chapters (from pledging to formals, in between their service
).
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SilverTurtle@greekchat.com
Phi Beta Fraternity
Phi chapter