Using testflight :
1) create the ipa file by development certificate
2) upload the ipa file on testflight
3) Now, to identify the device to be tested on , add the device id on apple account and refresh your development certificate. Download the updated certificate and upload it on testflight website. Check the device id you are getting.
4) Now email the ipa file to the testers.
5) While downloading the ipa file, if the testers are not getting any warnings, this means the device token + provisioning profile has been verified. So, the testers can now download the ipa file on device and do the testing job...