To be guaranteed a spot in res you need to apply separately, it's not enough to simply accept the offer. You are also guaranteed a spot if you have at least a 78% average (provided you apply).
You can apply here:
https://residence.mcmaster.ca /Default.aspx?Params=L 9ezxPcQnQvKnXsUHaBn65 ZMeJ6%2fb2QcWNX7VeZs0 4E%3d
There's a lottery system in place to determine which res you're in, but when you apply for res you can rank your preferences. When the lottery gets to you, if your first choice is available they put you there. If it's not they move onto your second choice, etc.
If you and a friend request each other AND all your preferences are the same, then they'll put you together.
You can check out the housing website for more information:
http://housing.mcmaster.ca/