I know they spent all IOTA coins for 500.000$. And everyone had to buy the coins with their own money. But how many people participated in the initial coin offering?
To my understanding, the question of how many people joined the ICO is hard to answer. Due to the fact that one person can have multiple addresses, I haven’t found any „official“ source giving out numbers of „people“. But you find a lot about existing addresses with a positive balance. Maybe that will partially answer your question. There are really well-written and worth reading articles about the distribution of IOTA on medium. For example here: IOTA Distribution: Update after Snapshot from 08th August 2017