Re-invite the user

in my project, when adding user to the group, one of them somehow make an error and delete the invitation email.
I re-invite the user but its always reject due the email already used. its not sent any invitation.
any idea or tips to solved this issue?

Hello. Try to remove the user and invite again.

I can not find the user name on the user list in my project, thats why I re-invite him again and got the error message

You can’t find him as he is not in your org. The user who created an account should remove his account by himself, only after that you can invite him.