Pop-up Box using OJS

Is it possible to have a pop-up box where readers can register their e-mail
addresses before accessing an OJS online journal article?

There isn’t a pop-up box, but in Journal Setup Step 4.1 “User Registration”, you can require people to create an account (including their email address) to view your content. See:

Some readers might see this as an annoyance, but it is one way to get them to register.

This is very helpful - thank you!