It gets pretty busy, pretty much everything out of Marylebone is busy at that time of day during the week. It will be somewhat quieter on a Friday than the rest of the week (there tends to be less business in London on a Friday so the train tends to have less people who are in London for meetings on board) but the usual tips for boarding busy trains at London terminals apply here. Try and meet the inward stock if you can, its a parping great class 67 with mark 3 coaches, it stands out like a sore thumb amongst all the 165s and 168s that normally populate the station. It usually comes in on platform 3 though.
People tend to be lazy and get on the first set of doors they see too, the coach nearest the barriers is always far busier than coach furthest from them. Walk up to the front of the train, if you are going to Moor Street then you will be closer to the barriers at that end when you arrive anyway. Dunno about the business zone, guards tell me that it is doing well if it is half full at the moment so you should be guaranteed a seat.
