It's not that McGill doesn't have a Greek life, it's that McGill has no incentive for a Greek life. On many campuses, frats and sororities are a way for students to move out of cramped dorms. At McGill, students move out of those after the first year, regardless. On smaller campuses, frats can be central to social life, as they have the means and the space to throw huge parties. At McGill, the city provides the social life. Some students do join frats, and those who do definitely love it; it's just that they're not a big part of the social scene. Why pay three dollars to wait in line for cheap beer when you can go out to a great bar and get good drinks? Frats do throw parties, but the people who usually go are those in another frat or sorority, or are friends with people who are.
What Greek life does provide for McGill students is a way to escape the anonymity of a big campus. People who choose to pledge tend to do so because they want to live with a tight group of people or enjoy being part of an organization. These are really good reasons, especially considering how impersonal McGill can be. If that sounds appealing, and you think Greek life will give you that, then you'll probably be happy. Just don't expect it to be the center of everyone else's social life.