Just an FYI for any who find this thread. When updating a multiple Person field, you use this pattern:
[ID0];#;#[ID1];#;#[ID1]
Where [ID] is the collection ID for that person. The tricky bits:
1. If you are only adding one item, and the Person's ID was say 2302, then your string would look like this:
2302
2. If you are adding two people and their ID's were respectively 2302 and 2441 your string would look like this:
2302;#;#2441
Good post about this here: http://pholpar.wordpress.com/2010/01/26/updating-multi-value-fields-using-web-service-call-and-batch-update/
From what I can tell, using -1 is no good.