Create external users using CSOM

Not by CSOM but you can invite a Guest user by using PowerShell along with Azure Function or Azure logic app service.

https://docs.microsoft.com/en-us/azure/active-directory/external-identities/b2b-quickstart-invite-powershell

Please see an example below:

private void ShareTheWeb(ClientContext context, List<string> emails)
{
     var users = emails.Select(email => 
         new UserRoleAssignment
         {
             UserId = email,
             Role = Role.View
         });

     WebSharingManager.UpdateWebSharingInformation(context, context.Web, users.ToList(), true, null, true, true);
     context.ExecuteQuery();
}

private void ShareTheDocument(ClientContext context, List<string> emails, string absoluteDocUrl)
{
     var users = emails.Select(email =>
         new UserRoleAssignment
         {
             UserId = email,
             Role = Role.View
         });

     DocumentSharingManager.UpdateDocumentSharingInfo(context, absoluteDocUrl, users.ToList(), true, true, true, null, true, true);
     context.ExecuteQuery();
}

Right now there is no way to add external user programmatically or using CSOM.The reasom behind it seems that SPO Webservice wont provide a mechanism to do this.