An user in my organization is not able to view data?
The user on clicking the invitation mail has been provided with a separate account and not part of your organization.
To verify this, goto Settings > User settings and check the status whether the particular user is in "Pending" state. You can find the option of Re-send invitation.
Click the Resend invitation and ask the user to accept this new invitation. After accepting the user should now be part of your organization.