I joined in 2016. I was reading a lot of Destiny news at that time, and a Forbes writer (Paul Tassi) linked here a few times. I followed the links and stayed around because of the sales info. I'm into business in general, and this is about as close as I've found to a gaming business discussion site for people that aren't insiders.
I was lurking for a year or two before joining. I created a different account maybe 3-12 months before this one, but I lost the password. So, maybe I really signed up in 2015.