Applied online for their summer associate position. After a very technical (it was even a bit pedagogical. I learned quite a bit during the interview) phone interview, I attended the on-site interview at the Stamford office and talked to 3 people. (one person was out of town at that time.) They asked mostly math questions, especially stochastic calculus. I couldn't answer all of them and felt bad but I eventually got an offer from them.

Round 1: Met with someone from a different IB group. Interview was unstructured and progressed based on my responses to interviewer questions about my group interest. Ended up becoming very technical for me, but I've heard the process varied for people across groups - with some applicants not receiving any technical questions.