Enable External Sharing on a SharePoint Online Site

Enable-SPOSIteExternalSharing.PS1

$tenant = "yourTenant"
$tenantUrl = "https://{0}.sharepoint.com/" -f $tenant
$siteTitle = "siteToShareTitle"
$siteToShareUrl = "https://{0}.sharepoint.com/sites/{1}" -f $tenant, $siteTitle


$adminSiteUrl = "https://{0}-admin.sharepoint.com/" -f $tenant



# As we want to check tenant wide settings then connect to tenant admin site
# Prompts for credentials. This is required if the account  using multi-factor authentication.
Connect-SPOService -Url $adminSiteUrl

# check tenant level sharing as whatever restriction is set here cannnot be lowered on the site we want to share below
Get-SPOTenant | fl SharingCapability
# SharingCapability : ExternalUserSharingOnly

# now check the site we want to share
Get-SPOSite $siteToShareUrl | fl SharingCapability
# SharingCapability : Disabled

# set this sharing to match the level set at  the tenant
set-sposite -identity $siteToShareUrl -sharingcapability ExternalUserSharingOnly
#
# Now we are able to share our site with our desired external users
#