If you're looking to meet more people outside of your rez, there are many options. Firstly, check out www.ssmu.mcgill.ca. There are a ton of clubs out there to join. Also, although sports aren't huge at McGill, I think it is very important for students to try out. As a rower throughout high school, I decided to take my first year off of the sport. However, in 2nd year I got sucked back in and made the varsity team. It was probably the best choice i've made. Some of my best friends are on the team. Check out www.athletics.mcgill.ca for all the info on varsity teams as well as intramurals - which are a ton of fun btw (1st year ball hockey team, 0-12, baby).
As for the night life. Monday night is A-Side (st. laurent), Tuesday night is BDP (ave du parc) or Cafe Campus (prince arthur), Wednesday night is benelux (Jeanne Mance and Sherbrooke), Thursday night is Tokyo (St. Laurent), and Friday/Saturday is everywhere. There's ALWAYS some type of party going on and you'll have no problem finding a place to go out for a beer (like i said, drinking age is 18!).