Collusion can occur in both online and offline poker. Back when I was just a newbie starting out (and did not give two hooots about poker),) we used to play in free tournaments at local bars. A couple of us would sit next to each other at the one table, and would use a bumping system; not the most minority system, but effective nonetheless.
One bump = I'm got a big hand, two bumps = I've got big pockets. The other person would bump back in a similar sense if they had a strong hand as well, and then we'd still raise to isolate our field and typically one of us would take the pot down. I still often go down to play at the bars and notice groups of other people doing it - in fact, some are even more audacious to the point that they're holding their cards at a high enough angle so their friend next to them can see what hand they've got. Nice.
I must be clear here, though: do not do it. You're ruining the game for everyone else!
Online poker collusion is a whole lot easier, and I'm sure it happens a lot. IMs just make it so easy to discuss current hands without any other eyes looking in. It's more effective than your typical offline poker collusion, in the sense that you can be sitting at opposite ends of the table, and still know each others cards. In fact, if PlayerA were to min-raise in 2 / 4NL with AA, PlayerB (on the opposite end of the table) could re-raise to $ 12 if a couple of players between them call, and then all of a sudden you 've got 2 or 3 players in the pot that are playing mediocre hands.
What can you do about it? Report them if you notice anything suspicious, and that's about it. Do not become a fish.