I have figured out my confusion. It is the links that do what I thought, not the emails. 
So the other way would be to create an invite link (rather than an email), and then send that in an email to all the people on your email list. New members would be taken to the ‘create account and accept invite’ page before being added to the group and sent to the topic (if you added that criteria), existing users who are already logged in would be added to the group and sent straight to the topic you pointed them at, and existing users who weren’t logged in can login from the ‘create account and accept invite’ page (though it is a little ambiguous whether you can do that, but you could add some extra text to the page in the js.create_account.subheader_title to add a little hint), and then be added to the group and sent to the topic:
However, this way also does not send a notification to say they’ve been added to the group, but you do get the opportunity to include an explainer in the initial invite email which might be useful? And the ‘arrive at topic’ could also be leveraged in a similar way as a landing page.
I’m not sure if that’s helpful, but I’m glad I figured it out.