Nearly two years ago, I wrote an article on Google's Policies on Pop Ups and Marketing Overlays and how it affects SEO. The practices discussed in that article still make sense for marketers today. Google's stance for desktop browsing is simple:
Google allows modal overlays that open within the same window that a visitor is browsing.
Google does not allow pop ups that open in a separate window outside the window the visitor is browsing.
This means that Google sees no problem with using pop ups within the same window to engage visitors. Presenting a pop up or overlay that provides relevant value to the content on the page is viewed as a responsible marketing practice.