They're the yellow shirt volunteers from this year's O-Week (of which I'm one). Each year the Vice Chancellor gives all yellow shirts a jersey as a thank you for their work during O-Week. This year it's the blue zip up jacket that you would have seen people wearing on Monday afternoon/evening.
It's also my 4th O-Week jersey.
For the second part of your question: yellow shirts are selected in session 2. The process involves a written application and a group interview. If you're interested, keep an eye out in the classifieds section of Blitz for an ad early in session 2 (usually in August).