But maybe the silent "w" is there in any case, and it really ought to be written - if it must be written - as "who' ", plural "who's"...
And then there's "who's" as a contraction of "who has".
And, while Dazbo's assertion Only plurals ending in "s" and, for some reason, biblical characters have the possesive [sic] apostrophe at the end of the word may be correct in Dazbo's vicinity, other regions of English (such as Oz, especially in universities) routinely use the possessive apostrophe with no subsequent "s" for any word ending in "s". As PoppaGator intimated.