Install / open SharePoint Online Management Shell
Connect-SPOService -Url https://grangerconstruction-admin.sharepoint.com -credential username@grangerconstruction.com
(login with admin-level credentials)
Set-SPOSite -Identity https://grangerconstruction.sharepoint.com/sites/SiteName -SharingCapability ExternalUserAndGuestSharing
(insert the site name with no trailing / )