It's intended to be a portmanteau of Group + Coupon, pronounced \ˈgrüˌpän
I've had an affinity for portmanteaus ever since I was 10, when I saw "portmanteau" in Through the Looking Glass and thought Lewis Carrol was talking about me. So I tend to be irrationally defensive of them.
My mind thought "Group Buying On _____," but now that you say it's actually "Group>o<n," I guess they mean "Group Coupon."