Can I add External users to a SharePoint group using (PnP) Powershell

I am unsure from your question that you are trying to add external users to SharePoint group / Office 365 group.

If it was SharePoint group you can use below PnP. Please note that the external users need to be present in Azure AD before executing this script.

Add-PnPUserToGroup -LoginName user@company.com -Identity 'Marketing Site Members'

If it was Office 365 Groups, AFAIK there is no PowerShell to add external users to the group.