Which option will be most cost effective? why?

Option 2 would cost more, because $42 for every 6 person, and there are 72 in total, so let's do the math. take the 72 and divide it by 6 to know how many pair of $42 you gonna spend. 72/6 is 12, and 12x$42 is $504. and the first option is $500 flat. so option 1 is the best option, cause at any time one of the guess might want to invite a random person, so you'd have to pay for every 6 person a guess on your list bring. while paying $500 flat wouldn't affect your paying rates.