They choose your groups but you can alter your timetable to a certain extent. You can go onto MyAdmin and alter your timetable online, but you can't exactly choose which tute classes you want to go to. You can have better control of exact tutes by going to change your timetable at The Student Center (at the bottom level of the Carslaw complex on Eastern Avenue), although whether or not you get into the tute time you request depends on the current number of students already allocated to that time, amongst other things. Sometimes if you miss your tute, you can on notification to your tutor go to another tute to finalise that week's work. However, for many subjects you MUST attend all tutorial quizzes/exams at the quiz you've been allocated to.
For science, the tutorial attendence rate [from memory] is around 80% or something for alot of subjects (or else they could fail you at their discretion), however I only went to about 50-60% to some and they didn't really do anything.