Yes, the people who have booked with guides get priority. Other people just have to turn up on the day and hope there are still enough places left as private individuals can't book in advance. It's not a problem out of season (I was the only person who went up in 3 days last week! It was bad conditions - I do a lot of solo mountain walking in Scotland but would have struggled without a guide in the thick cloud and ice/snow as I had no map, compass etc ), but in July-August, the 200 places can be all allocated by 6 or 7am apparently (it includes people who are still on the mountain from overnight climbs). Even if you get a place and can go up, in summer you'll be queuing at the summit I suspect! Note there's no public transport to the mountain either, so you have to pay for a taxi there and back if you don't have a guide.