Collusion happens very often. I had two friends that would do it, and I've even done it with them, playing high stakes S&Gs together. Just try to keep an eye out for it. For example, I always check where everyone is from, and will leave a table with multiple people living in the same home town.

There isn't much online poker sites can do to stop this from happening. There are just too many ways for people to communicate to stop. Like stated above, reporting players is probably the best defense, but even then, it's hard to prove.