This just came up again here, and I have to admit I have to look it up myself every time it does come up! So - for future reference, if you need to submit data to a person or group column in a SharePoint list, you can submit -1;#userid with userid replaced with an actual user ID. If you need to submit multiple values, separate them with ;#

-1;#user1;#-1;#user2

And so on...

SubmitToSharePointList submits CAML to the UpdateListItems method of the SharePoint Lists web service, so if you are submitting using your own custom code, or what-have-you, this will work for you too.