It isn't directly addressed, but if you look here here you can see that James Jacobs seems to accept the notion that cackle could work on a whole party.
Unfortunately, that is the only developer word I can find. I'd assume, since most people seem to assume it works on everyone, nobody has bothered to get it directly addressed.
There's no need for official clarification. It says "any creature" with no indication of restrictions. The hex is not formulated in the way that effects that only affect one creature within a range are worded. If the intention was only one target, they would've directly said so.
This is not a game of subtleties.
Also, Jason B, SKR, SMR, and Mark Moreland all have not commented on cackle.
from the link Mort gave "A witch's cackle only affects targets within 30 feet" multiple.
Also in my english understanding (not native) any means every, or else it would have to be targeted, which is always specific "target creature" or say something about which creature is effected first.
You're right though that "every" would have been even more clear.
Cackle works on as many hexed creatures as are in range. The wording isn't explicit, but it's clear from the use of "targets" in the description.
Also, you can cackle twice in one round if you use your move and standard action to do so, and that extends the duration of hexes by two rounds, freeing your witch up for a full round action in the following round.