Hi @Mark_Gaffney, welcome to the Asana Community Forum! Our support team can help you here. I recommend you to request to the user who lost access to contact our support team confirming the name of the Organization or Workspace and the name of the project they lost access to. If they have an email notification, they can try to copy the URL of the project from there and sent it to support as well. They can help them recover their access!
Let me know if you have any other questions and I hope the issue is solved soon!