Hi Matt,

The best way is a custom task or a modified version of the existing CreateMemberForCustomerTask in the checkout pipeline.

The trouble is that by default passwords are encrypted in the database, so you'll have to gen the password and e-mail it one go.

The CreateMemberForCustomerTask in the checkout pipeline already does this, but obviously doesn't send out the e-mail. What you can do is inherit the existing CreateMemberForCustomerTask and override the GeneratePassword method on the task.

I whipped up a sample to do it. Basically what it does is:

The CreateMemberAndSendPasswordTask class inherits the default task and overrides the GeneratePassword method. Because the key method, Execute, is not marked as virtual I'm using two classes to make it go: 1) A shell that only executes my custom task, and 2) the actual custom task, which overrides the GeneratePassword() method and sends the e-mail using the configured template. 

public class CreateMemberAndSendPasswordTask
{
    public PipelineExecutionResult Execute(PurchaseOrder order)
    {
        // New an instance of our custom task
        var task = new CreateMemberAndSendPasswordInternalTask();

        // Save the order for use in GeneratePassword to get the e-mail
        task.PurchaseOrder = order;

        return task.Execute(order);
    }
}

public class CreateMemberAndSendPasswordInternalTask : CreateMemberForCustomerTask
{
    public PurchaseOrder PurchaseOrder { get; set; }
    public override string GeneratePassword()
    {
        string password = base.GeneratePassword();

        var customer = PurchaseOrder.Customer;

        // Build a list of parameters to pass to the e-mail service
        // Parameters are passed to the configured e-mail template with the key specified, 
        // e.g. MyWelcomeTemplate.apsx?password=<generatedPassword>&firstName=<firstName>&lastName=<lastName>
        // Can also be used in the subject in the form of {password}, {firstName}, {lastName}
        var parameters = new Dictionary<string, string>();
        parameters.Add("password", password);
        parameters.Add("firstName", customer.FirstName);
        parameters.Add("lastName", customer.LastName);

        var emailService = new EmailService();
        var emailProfile = SiteContext.Current.CatalogContext.CurrentCatalogSet.EmailProfiles.Single();
        emailService.Send(emailProfile, "WelcomeEmail", new MailAddress(PurchaseOrder.Customer.EmailAddress), parameters);

        return password;
    }

}