Como se dice opt-in en español?
por ejemplo "large database of opt-in subscribers"
2 Answers
While I don't disagree with ChamacoMalo regarding his answer, I think that a more accurate way to describe "opt-in subscriber" is someone who has "explicitly given consent to be subscribed to a service, publication, etc.(i.e. the subscriber must make a concrete action, like going to a public library or requesting and mailing in a form to become a subscriber)
This is in contrast to opt-out subscriptions that automatically subscribe people unless they "opt-out" or indicate/state explicitly that they do not wished to be subscribed. This can also be called "presumed consent" as subscribers are presumed to consent to subscribe to the program, service, etc.
Because opt-in/opt-out has to do with consent rather than actual enrollment (subscription), I suppose that something might be constructed such as consenting subscribers for opt-in subscribers.
To describe a consenting (opt-in) subscriber perhaps you could say something like
Una persona que consintió en subscribirse a algo - a person that consented to be subscribed to something (a newspaper or magazine, etc.)
Una persona que consintió en dar donativos a algo/alguien - a person that agreed to make contributions to something/someone/a group (charities, etc)
Una persona que consintió en abonarse a algo - a person that gave their consent to sign up for some type of service (telephone, internet, etc)
Opt-in is to subscribe. Your word of choice would be subscribe. In spanish, that would be redundant, as it would be in english, since a subscriber has already "opted-in" 
