It isn't quite common, but many of the internet darlings did this same thing. I don't know why it is news all of a sudden. While GRPN has the smallest float, there were many other companies in the ballpark (P, LNKD, GOOG, etc). A more general rule of thumb for IPO's is 20-25%, but all of these guys had single digit values. It is fairly smart move on their part. They get a higher valuation due to demand and supply, and a long as all of the investors/employees don't flood the market with their shares, the early investors/employees win.
Recall Google: http://en.wikipedia.org/wiki/History_of_Google#Financing_and...
They floated 14m out of 271m total shares, and other stockholders added another 5m or so... so the total amount it IPO'd for was around 7% of the company but GOOG's share was roughly 5%