A family of Microsoft on-premises document management and storage systems.
These users do not necessarily need an office 365 account.
You can enable anonymous access for them and invite external users to visit your site, but they need a Microsoft account. When you share your site with external users, they will receive an email prompting to log in with a Microsoft account or a work or school account in Azure AD from another organization.
Information about External sharing:
https://learn.microsoft.com/en-us/sharepoint/external-sharing-overview
Sharepoint online subscription information:
https://www.microsoft.com/en-us/microsoft-365/sharepoint/compare-sharepoint-plans
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