flirt, the Panhellenic Association for NMSU does indeed have bylaws.
Go to this page, scroll down to Panhellenic Council Resources, click on "Panhellenic Council Constitution". I just skimmed the bylaws and find nothing about social media and mandating the Greek Life Advisor's participation in same.
I could see adding a specific chapter advisor to that sorority's account; plenty of reasons to do so. However, involving non-chapter members in "chapter business" makes me really uncomfortable. Frankly, this is something to take up with AXiD headquarters, for starters. I'd reach out to the other NPC sorority advisors as well because I suspect you aren't the only one questioning this "policy". And no, this isn't a mandate from NPC HQ (to involve Greek Life personnel in social media) unless something's just changed.
Hybrid Mode
