Asked and answered
here.
You're right, a playlist cannot be grouped in Theatre View. It's not possible given the current design. But a playlist is a playlist, not something to be used to define a view. A much more effective approach is to create a view based on the same expression as a smartlist—and then any categories may be added to provide the desired "grouping." Such views can also be nested, so it's very easy to create a variety of different ways to categorize and group a collection of any size.
To illustrate, I use expressions to create views of "movies not recently viewed," "movies viewed" (sorted by date and grouped by month and year), "videos recently added," etc. The possibilities are endless. I even have a set of views to display lists suitable for different types of viewers (i.e., men, women, children) based on several attributes. For example, for men...
(([Genre.]=[Action],[Crime],[Horror],[Sci-Fi],[Science Fiction],[Thriller],[War],[Drama] and -[Genre.]=[Comedy],[Family],[Fantasy],[Musical],[Romance],[Spirituality] and [Tones.]=[Angry],[Biting],[Creepy],[Cynical],[Disturbing],[Downbeat],[Earthy],[Eerie],[Explosive],[Forceful],[Futuristic],[Grim],[Gritty],[Gruesome],[Haunting],[Ironic],[Lurid],[Macabre],[Menacing],[Nocturnal],[Ominous],[Paranoid],[Racy],[Raunchy],[Rescue],[Rousing],[Rowdy],[Tense],[Upbeat],[Visceral],[Wry] and -[Tones.]=[Affectionate],[Campy],[Compassionate],[Endearing],[Fanciful],[Gentle],[Heartwarming],[Intimate],[Meditative],[Passionate],[Poignant],[Sensual],[Sentimental],[Sweet],[Talky],[Tearjerking],[Warm],[Whimsical]) or [Moods.]=[Blood and Gore],[God and Country],[Thrill Rides],[Tough Guys],[Mindbenders],[Nail-biters],[Other Dimensions],[Triumph of the Spirit])
This, of course, isn't very practical. But it illustrates the power of using expression views in Theatre View. And my female visitors are always impressed when I surrender the remote and say, "Go ahead, select
Chick Flicks."