You'll need a single opt-in form at the very least to use ActiveCampaign’s hosted email marketing service, although I'd recommend a double opt-in.
Automatically adding users is not the best way to go about this imho.
For that you'll have to go to your activecampaign dashboard, choose the form and click in integrate. At this point you can use the wordpress plugin, or, if like me, you'd like to have a little more flexibility you can embed the form using the code provide.
You can see a working example scrolling down one of my client's homepage:
http://lumeo.com.br/
Hope it helps