This works for both rich InfoPath forms and web forms, not sure if this will work for mobile forms.

Follow the steps exactly, including the name of the group and fields with EXACTLY the same case (it is case sensitive)

http://blogs.msdn.com/infopath/archive/2007/02/28/using-the-contact-selector-control.aspx

The instructions give you the steps for adding one field only, but if you want to add more than 1 field then you do the following:

1. go to the Data source pane and add a new group for the field that you want to capture. In my case Approver 3.

2. Go to the gpContactSelector field that you originally added and select “Reference”

3. Then select the group that you just added (in my case Approver 3)

Now you have multiple people picker fields.