We have > 120 mentors across a number of domains, industries, companies, and even locations. Many of them are founders/CEOs of our own portfolio companies. The mentors help our startups by speaking at our weekly talks, holding office hours at the space, advising startups 1:1, contributing content for us on the 500 Blog, referring cool startups to us from around the world, and more. Plus they have secret powers.
As a startup not (yet! ; ) in the 500Startups program, I've been exploring this question myself. If you are a startup, and are interested in the 500Startups mentor program, here is what I've been able to piece together from the various mentors I've spoken with:
Founder reaches out to mentors through founder's own network
Some mentors may be interested enough in founder/startup to take a meeting
Mentor might then start providing mentoring (but not as part of the "500Startups Mentor Program"), or may refer startup to Dave McClure
Startup may become a 500Startups startup - this decision is made by Dave McClure and his inner circle
If selected to be a 500Startups startup, which typically (always?) includes investment by 500Startups/Dave McClure, the startup will start receiving referrals from Dave and his team to various 500Startups mentors for short or long-term mentoring on specific business issues
Additionally, as noted by Christine, once part of 500Startups, the startup may attend presentations given by 500Startups mentors
Mentor secret powers may lead to additional weird and wonderful effects, not as yet fully understood by anyone