Guest users

Guest users, as a concept, are more closely related to SharePoint and OneDrive. A guest user has been granted access, via sharing, to a resource on SharePoint or OneDrive. They're only relevant if your organization allows external sharing.

A guest user will automatically be created if you create a sharing link for a specific email address within SharePoint or OneDrive. You can't create them here, but you can view and delete them.

Note that guest users don't have a presence in the global address list, and the same email address can't be both a contact and a guest user. If you have a need to give people who are frequently contacted by your users access to SharePoint and OneDrive while also having them as a global contact, and also having them on a list that automatically sends them and other people email, it might make more sense to use an Office 365 Group rather than a traditional distribution list, because members of those Groups can be both guest users and mail contacts at the same time.