A platform designed to support both: subscribers and user accounts?

We've created a platform where users can subscribe to their favorite organizations' newsletters (similar to Mailchimp). But we also want to allow subscribers to post content on their organizations' page (similar to Yelp or Meetup).

We're afraid that if we were to force everyone to create accounts, it might deter those that are new or just want to receive the weekly newsletter to sign up.

We're in the early stages, and our focus is to expand our user base as much as possible.

Are there any past lessons or existing examples that we can learn from?