I have no way of knowing which coworkers contributed to the gifts (nor do I think it appropriate to ask). One coworker in particular was the organizer and spearheaded the whole shebang.
Bold above is the reason why I'm going with a group TYnote.
I was once a recipient of an individual TYnote from a bride-to-be where I did not contribute to the gift. It made me feel very awkward. In fact I posted about it here wondering what to do.
An additional note to the Organizer? I'm not sure, it depends on how much extra the organizer did. It's kinda like a special thank-you to the hostess, which would be the right thing to do. If she collected the money, bought all the gifts/decorations/food, set the table, etc. then she deserves a special thank-you.
But you have no way of knowing exactly who did what, right? Maybe one person went to this store, another went to that store, maybe everybody brought in food/cake (if that was even done)?
One heartfelt TY to everybody is the way to go.