The first few days of welcome week are mainly for new students, however lots of activities continue into the first week of school and everyone can get involved (i am not talking about the special first year events that you need the macpass for, just the general ones during the first 2 days of school). Regarding transfer students, i think they would let you join since you are a new student, even if you are transferring into second year.
I am not sure if they released the schedule for welcome week yet, but you can check these links in the next couple of months for updates:
https://www.facebook.com/MacWelcomeWeek
http://studentsuccess.mcmast er.ca/...come-week.html
I know that the first one is for the 2012 one, but they might release the schedule there. You can also keep checking Macinsiders because they usually post the schedule here.
Welcome to McMaster University
