Here's the reply I received from the monastery:
We normally provide accommodation for pilgrims in their way to Santiago de Compostela. They spend the night in a building called “Nostra Senyora”. The price for them is free. In the case you are not pilgrims going to Santiago, you should pay 10 € per night.
The rooms are very simple. They only have 2 or 3 bunk beds and small wardrobes, and obviously toilets and showers. Bed linens and towels are not provided. You have to come with your own sleeping bags and towels.
To collect the keys, you should come to our office, the Pastoral Coordination Centre. Our timetable is as follows: from 9,30 a.m. to 13,00 p.m. in the morning and from 16,00 p.m. to 18,00 p.m. in the afternoon. We open every day of the week, including Saturdays, Sundays and holidays. If you wouldn’t be able to come within the office timetable, we could leave you the keys at the reception of the Hotel Abat Cisneros. They are opened the 24 hours, and the hostel for pilgrims is just 1 minute walking from the Hotel. You cannot get lost. Our telephone number is: 93 877 77 66
When you know the exact day you will como to Montserrat next year, please let us know in order to book one of the bedrooms for the members of your family.