Add a user (editor) to a project

[makatowicz]

I have added a user to a specific project. Mail is not recieved by the added person. Tried two times. Then watched a youtube video and confirmed that they do it exactly the same way as I did. Why is email not received and why can the person still not sign in. 

[Penelope]

Are you by any chance referring to the Google Cloud IAM? I don't know the answer however I found some documentation about troubleshooting located at https://cloud.google.com/iam/docs/troubleshooting-access, which might help you to figure out if they do have access to the project or not.
The page on how to manage access to projects, folders, and organizations: https://cloud.google.com/iam/docs/granting-changing-revoking-access 
Some just common sense things to try would be to check and verify you have added the right email address (and checking to see if the person is part of the same domain and if so, then add in their email address that is inside of the domain rather than an external email address), if the user does try to log in what is the error message and that can help guide you on what might need to be done based upon that error message, also checking the permission of yourself meaning do you have a high enough permission to be trying to add users to projects, and if you are not the admin then try reaching out to them for assistance (especially if this thread gets no help).

[lordiron]

It looks like a bug. Sometimes it happens because of a read failure in the database or because the person can't read if something is wrong with their email. I experienced a similar inconvenience a couple of times when I converted a video and wanted to email it to several people, but an error would appear. I couldn't solve it, so I had to convert the video via loader.to and email the video itself to people in mp3. After that, everything worked, and I had no problem with video conversion, so I didn't bother with it.