I still think the beer
fund is the way to go, non-refundable
Or simply for a last night meal ashore (so you can make arrangements in advance).
No reason not to be upfront with folks by telling them that it is simply about obtaining a firm commitment that you can plan around (for own benefit and for the other crew / guests). For first timers can explain that all
the crew chip in the same (you included) for the beer
/ meal ashore so they are not being singled out - how much in advance that other crew chip in is another thing
No guarantee that they will arrive after chipping in only $20 or $50 (my bet is that most will) - but at least your dissapointed can be tempered by a couple of rounds from "absent freinds"