In Nucleus One, in the Organization Members Page or Project Members Page, confirm that the user's email address is listed. If so, the Nucleus One invitation email may have been delivered to another Incoming Folders. Please ask the new member to check their Inbox, including other Incoming Folders like Spam folders.

To to go the Organization Members Page, “User Menu” > “Manage Organizations”. Click on your Organization.

To to go the Project Members Page, “User Menu” > “Manage Organizations”. Click on your Project.