Well first of all remember CS community wasn't what it is now.
So basically SK insider was a mix a faceit, VIP access on the SK website and you could get reward.
You had a IRC channel where insider and SK players stayed. You could !join a game and when you had 10 people it'd launch a game (like faceit solo queus, but with the same people every time pretty much, like a comunity). There was also AIM server, CSDM servers (where you would find some pro) and KZ server.
The most you played, the most point you'd get on the "insider ranking". I remember joining KZ servers and idle all day at school. I eventually won a 5hv2, few mousepads, 1 free SK insider year. Also every month the top ranked players would play pug with SK players.
You had access pro players demo with the in game comms and ingame chats (which now is basically watching as stream on twitch)
You would have guide about CS: the spray patterns, the moves, video of SK on LAN, gaming house and some tactics from the team, the why's and shit of calls. That was like "the thing" at the time but nowadays you have that on youtube for every team.
And you got a nice orange cube on your SK profile which was liiiiit.
SK found a way to make money of fans was before everybody