Sure, "We are Groot," but it takes a special person to actually become Groot -especially in a full-sized costume. On Instructables, you can learn how to become just such a special person by making your own cosplay. Note that when I say "life-sized," I mean life-sized based on the film, not human-sized -meaning 7'7".
Of course, the project won't be easy, but you might be surprised at how cheap something this incredible can be. And, the end result is certainly well worth the effort.