Depends what you mean by rooftop. If you want a real elevated deck with seats, the walk-in window is earlier than 6:30 on a decent weather Thursday. The places by Gantry fill by 5:30 or 6 in my experience. I've had luck at Anable Basin around 5:45 on a Tuesday or Thursday, but not on a roof—more the outdoor patio by the water. It's not high, but you're outside and there's usually a bench if you circle.
For actual rooftops, the hotel spots around Queens Plaza are more loungey and go reservation-heavy when the weather is good. I wouldn't send a group there without one person holding a booking.
If you're okay with a backyard instead of a roof, LIC Bar has a small back area and is the most walk-in-friendly in that pocket. Dutch Kills is excellent, but it's a cocktail room—low-lit, no view, and small. Good for two, not four coworkers standing around.
My honest suggestion: do a two-stop plan. Start at Fifth Hammer for a first round while the sun is still up, then check the second-floor deck at The Last Word or the patio at Alewife as a backup. None are true rooftops, but they scratch the outdoor itch without a rope line. If someone insists on elevation, you probably need either a 4:30 walk-in or a Thursday reservation.