Hi @Taskle,
You are not able to invite him or add him manually because he has never logged in to SC. When syncing members from GitHub, we check if the user already exists in the SonarCloud, if he does not we do not create an account for him. The only required action is that the user in question has to log in first to SonarCloud with his GitHub account. It is not required to invite beforehand. You can find more information here: https://sonarcloud.io/documentation/organizations/manage-team/
Let me know if have any further problems
Best,
Marcin