But perhaps I should add a not in the sign up agreement that "by signing up you agree to receive emails from us" or something like that, to be safe.
If you're in the EU, this wouldn't constitute as opt-in. The EU states that a user must specifically click a button that states "I want to receive these emails". Unless it's regarding their account. So, if the daily digest included notifications, it would be fine, but if it was just new topics, I'm not so sure. It should be fine either way, as you've said.