Selling employee vested stock privately to current investors

I keep getting contacted by former coworkers (engineers) who either recently left, or, who are miserable and want to leave somewhere where I used to work. The common thread is they cannot afford to exercise their vested options and pay the tax so they are letting options expire asking me for advice since I used to work there. Is this really common among other tech companies? Shouldn't the current investors (institutional or otherwise) have some interest and ability to buy up employee vested stock?