You can choose to identify which iPhone Squirrel Street app user submitted a document by turning this function on in their iOS app.  Here's how:

1) Login to the iOS app as that user and click on the user name

2) Swipe right as shown to save the submitter.

After this, whenever a document is submitted using that user's app, a tag with "Submitted by [username]" will automatically be added to their submission.  Once the document is in the account, you'll be able to filter for this submitter as you would for a category.