I assume there are some conversion-optimization experts who think and consult data to choose the ideal timing and frequency to invite a visitor to sign up. I.e.

I assume there are some conversion-optimization experts who think and consult data to choose the ideal timing and frequency to invite a visitor to sign up. I.e. the moment they are most likely to do so if invited. ... and the other 95% of folks who just grab an off-the-shelf email-list-invite popover widget and slap it on a web site.