I'm attempting to allow external users to access a SharePoint site for which I am an administrator. The SharePoint site is set to be shareable with external users. All of the users in question have the same email domain (@company.com). Some of them have company-provided Office 365 accounts under their company email, but others do not.
In order to grant these users access, I have added their email addresses to a SharePoint group that is permissioned to the site (it has "Read" "Edit" and "Contribute"). Some of these email addresses autocompleted to the users' names, while others remained displayed as the bare email address. This appears to correspond to which ones had company-provided Office 365 accounts.
A few of the users (I believe the ones with Office 365 accounts) were able to access the site. Unfortunately, most of the users I attempted to share the site with this way were unable to access it, getting the following error:
This Microsoft support page suggests that SharePoint sites can be shared with users via their Microsoft account, so I had the users without Office 365 accounts create Microsoft accounts under their existing emails. However, even after doing so, and re-adding them via their email to the SharePoint group, they are unable to access it.
Is there a reliable way for me to allow these external users to access the site?
(Please let me know if there is additional information that would be helpful in answering this, and I'll edit it into the question.)