It could be a variety of things. Is the user receiving any emails from Atlassian at all?
Check the Notification Scheme to be sure the user is incorporated to whom should receive emails for different events.
Have the user check their Personal Settings to make sure everything is checked as needed:
We’re excited to introduce external collaboration for Confluence, now available in early access. It is available to preview for Confluence Cloud Premium and Enterprise customers. (If you're not on ...
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