Ios add device to provisioning profile and build using testflight

1. go to TestFlight

Manage People in <Project Name>
Check off the people you want (should all be iOS 6 people)
In the upper-right >> Export Devices
--> this will save a txt file (make sure just 1 device per person in this)

1. go to http://developer.apple.com/ios

log-in to account
go to iOS provisioning portal
go to Devices
click on Upload Devices (open the txt file from above)
go to Provisioning
click on Distribution tab

on provisioning profile (will have to create if this is the first time building), on right side, click Edit -> Modify
  check the people you just added
  click submit
refersh until it changes to Active
Then Download

1. in TestFlight

Builds >> on the build you want to update >> Edit
in Permisssions  >> Update Profile
   then upload the file you downloaded above 
Once the provisioning file is uploaded 
 click the check box on the left for each of the newly provisioned testers 
 scroll down the page and click 'update & notify"