Do people who schedule a reminder become newsletter subscribers ?
When your visitors ask for an email reminder through a Remindee popup, they become your subscribers, not Remindee subscribers. Remindee is just a third-party that captures email addresses and send them 1 email reminder, on your behalf.
So, technically these subscribers are yours and you are free to download their email addresses and send them newsletters.
However, in the EU, you are supposed to explain to users how you plan to use their personal data. If you don’t explain it in your popup, users might think that by asking for an email reminder they will only receive a reminder, nothing else. That’s why we recommend to websites with UE visitors to display GDPR compliant popups, so your subscribers will be fully aware that by asking for a reminder they also accept to receive your newsletter.