It was certified as a class action lawsuit. I'm not a lawyer, so take this with a grain of salt, but from what I have seen of such things in the past, you would typically have three choices:
1. Contact the appropriate authority to opt out and preserve your rights to sue individually (or not at all);
2. Do nothing, and if the suit wins
a. contact the designated organization with proof you were affected to receive your share of the judgement, or
b. do nothing, and you don't get anything, but your name never shows up anywhere.
However, this is Quebec, and furthermore involves a company that keeps detailed information on everybody who purchases its services, so I am not surprised that you have to act now to opt out. On the other hand, the class is defined in such a way that proving that you were deceived might involve forcing Air Canada to cough up old website information to see what it displayed for a price the particular days you purchased tickets. Or the court might say "screw that; the cost of doing so could easily eat up the entire award, so everybody in Quebec who bought tickets during that time is automatically a winner."
As far as being included without having been asked, that's the point of class action suits. The lawyers for the person seeking certification propose conditions to determine who is included, and if the judge decides that they're reasonable (for instance "every person who has ever lived in Quebec" is not logical since many or even most of them haven't flown Air Canada and many who did didn't travel during the time period specified).