Hi, @Glenn Maxwell
The answer to Andy is the best direction.
After my testing, this works.
You can use the following command to invite external users:
New-MgInvitation -InvitedUserDisplayName "FL LN External" -InvitedUserEmailAddress someid@gmail.com -InviteRedirectUrl "Your RedirectUrl"-SendInvitationMessage:$true
It should be noted that you first need to install and import the Microsoft.Graph and Microsoft.Graph.Identity.SignIns modules.
The test results are shown in the figure below.
You can see in Azure AD that the external user you just added was successfully added.
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