Most of the technical work has been done by Jochen Theodorou. Others who have helped technically are Paul King, Cedric Champeau, Alex Tkachman, and Jeremy Rayner. James Strachan also did a lot of technical work on Groovy in his day. I'd rather listen to what someone who's done technical work has to say than someone who shows powerpoint slides.

You mentioned Guillaume Laforge as leading the Groovy project, but from my experience, Graeme Rocher, the Grails Project Manager, seems to have far more say over Groovy's direction.

As for calling Groovy "one of the most popular languages in use today", it figures fairly low in most popularity ratings (e.g. not in Tiobe's top 50), and only shows the occasional burst when it's been actively talked up, e.g. just before a conference when the project managers are selling conference seats.