Hi ken fueki,
I am glad to be able to help you : )
Typically, you can invite external users to your site form Site permissions, and you can refer to the last additional steps.
Since at this time, we couldn't be sure if you're following these steps to invite external users. We couldn't infer which step in the process went wrong and caused the error message "Sorry, you don't have access".
So if possible, would you mind providing a more complete process (includes information such as whether external users receive the message and whether they can open the invitation link) on how you invite external users. This way we can better troubleshoot.
In addition, you can also check whether the browser's cache caused the lag problem that causes the inaccessibility.
Steps to invite external users:
1.Click the Setting in the Upper right corner, select the Site permissions
2.Select the Advanced permissions settings
3.You can invite external user to the corresponding group according to the permissions you want to grant, click one of the following default groups.
4.Click the New button and enter the external user's email address. Then an invitation email will be sent. You can add the user as instructed by the email.
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