Convert Azure Active Directory AAD User from Member to B2B Member. Associated blogpost https://blog.darrenjrobinson.com/convert-azure-ad-users-from-members-to-b2b-members/

B2B Member Invite

# Invite User Request
$inviteDetails = @{
    "invitedUserEmailAddress" = $externalEmail.otherMails[0]
    "invitedUser"             = @{"id" = $updatedUser.id }
    "sendInvitationMessage"   = $true
    "inviteRedirectUrl"       = "https://myapps.microsoft.com?ResourceTenantName.com"
    "invitedUserMessageInfo"  = @{
        "messageLanguage"       = "en-AU"
        "customizedMessageBody" = "Hi $($updatedUser.displayName), Your user account '$($updatedUser.userPrincipalName)' in the Contoso Azure Tenant has been migrated to a Guest account federated to your Fabrikam '$($updatedUser.otherMails[0])' account. After selecting 'Get Started' below you will be able to access Contoso using your Fabrikam account and credentials. All you access and privilages remain intact. Regards, Contoso IT"
    }
}
$inviteBody = $inviteDetails | ConvertTo-Json

$invite = Invoke-RestMethod -Headers @{Authorization = "Bearer $($accessToken.AccessToken)"; "content-type" = "application/json" } `
    -Uri  "https://graph.microsoft.com/v1.0/invitations" `
    -Method Post `
    -body $inviteBody

$invite