Enable external users to use Approvals in Flow
As explained by Microsoft Office365 Support, the template Approvals in Flow does not allow external users to interact with the full process. An external user can access a SharePoint site (e.g. to browse an item list) but he/she cannot press the button Approve/Reject in the email created by the Approval process in Flow. This restriction "by design" is negative for companies like ours where we handle approvals of documents by colleagues who work for different subsidiaries (=every subsidiary a different Office365 tenant) and therefore it is impossible to manage flow inside such organization.
Lisa Grewe commented
I have just encountered this issue. Our organization helps to manage a non-profit - with board members from other organizations with their own email addresses. They are on our SharePoint site as external guest users and we would like to use the approvals Flow to get documents approved by the board and committee members.