To send Testflight invitations to users you must have:

• Reviewed and Approved iOS app 
• iTunes Connect
• Email Address of recipients 

To send Testflight invitations:

1. Login to iTunes Connect
2. Click My Apps 
3. Click on your app entry 
4. Click Testflight 
   
5. Click on your external group or iTunes Connect Users 
   
6. Click on the + symbol 
   
7. Click Add Testers 
8. Enter the email of the recipient and first name and last name (optional)
   
9. Click Add 

An email will be sent to the recipient's email and look something like this:

Alternative Method

1. Ensure the iOS app is reviewed and approved.
2. Enable the public link for the app in the app entry in TestFlight under external groups or iTunes Connected Users.
3. Copy the link and share it to the recipients.
4. Ensure that the recipients know that the link is public and anyone with the link can open the application.
5. The recipients should know that the link has to be kept confidential.
6. The link opens up to the page with complete instructions to download the app through TestFlight.
7. The link should open up to a page like this with instructions and next steps.