Some users can't be add to my appstore connect account

I'm adding many users to internal testing purpose. A few users did't received and welcome to apple developer program after signing in from invitation email. I've checked and didn't they on the user lists, so I can't added to internal tester group. We've tried so many times. I noticed that every users that had the same problem didn't see the trust this device page during the signing process. How should I fix this problem.