Hi Ryan,
I think this is a limitation of the Contact Selector. We created a separate contact selector that uses Active Directory and supports search. You could use that to promote multiple users, but you'd have to have separate fields in your form for Contact1, Contact2, etc. and set those to the indexed values in the repeaintg group: my:repeatingGroup[1]/my:contact, my:repeatingGroup[2]/my:contact, etc. Then, you promote the Contact1, Contact2, ... values to your SharePoint columns.
SharePoint columns don't handle repeating data very well, and I would recommend looking at the SubmitToSharePointList functionality in Qdabra's qRules or submitting the values to a database (via a tool such as Qdabra's DBXL) and setting up a data view to show contacts as sub rows underneath a primary row for your form.
Good luck!