Hi @Ian_Atwater , I’m probably not the best person to ask. I’ve just been using the GMail pack to email a page which has a lot of inline formulas and filtered tables (based on a control value, in your case this would be to select the new user, or maybe just their email address).
So I’d start with a template page that has the content you want to send and use inline formula’s to customise things like the user’s name and any other info specific to them. The only issue is that you don’t have much control over the styling of the email - that’s why my next adventure will be looking into custom html and css.
@Paul_Danyliuk 
has a great reference doc on this here.
@BenLee also has a great one here.
Hopefully that helps a bit!