Presumably they are suing the Board members personally - not the Boeing Company - since as noted any payoff from Boeing to the shareholders would be silly.
Problem with that is, Board members are basically protected so long as they were acting legally and in good faith - the company has to defend them and would be responsible for any damages (I have some familiarity with this, the board having been sued when I was on the Board of my racing sanctioning body). Now, if the Board members broke laws or engaged in some sort of fraud, all bets are off - but that would be extremely hard to prove. Plus, while I'm sure the Board members have deep pockets, I don't think they are anywhere near that deep. We're talking several tens of billions of dollars of stock value - total net worth of the Board would be a small fraction of that.
Perhaps WillowRun will step in with a more legalistic take on this, but my take is this is some shareholders having a hissy fit that will simply make lawyers richer at the expense of Boeing and it's shareholders.