I'm not sure how they go about picking these people. I imagine they select people that are personable, and at least respectable if not exceptional students.
The students I spoke with gave me some great insights, and lots of random anecdotes that I was able to use in interviews. Your interest in a school seems much more sincere if you can manage to weave these sorts of things into an interview. For instance:
INTERVIEWER: Why are you interested in our school specifically?
YOU: Well, I'm interested in advertising and I know Kellog has the preeminent blah blah blah...and also for reasons X, Y, and Z, and the students I spoke with were all very enthusiastic about the travel program, in fact one of them told me that when they were travelling to England they met Richard Branson face-to-face, which is unbelievable, but I know that these are the sorts of amazing opportunities I could look forward to at a great school like Kellogg...
Dropping an anecdote like that will (1) Show that you took the time to reach out to students (2) Show that you are personable (3) Will make you a more memorable interview because you're talking about something interesting instead of saying, "I have great quant skills, your school will help me make them even better, I love numbers, I'm a robot" (4) It will show that you have "awareness" or "maturity," you're able to recognize that b-school is more than just classes and placement statistics, you will come across as someone who is genuinely excited about the b-school experience.
Your initial questions with an ambassador might not yield much, but if you follow up with them and probe to get some interesting stories/insights.