Are there four or five of you in this group?
 
That makes a difference as to what to suggest.
 
What cities are you visiting?
 
For a fair-sized family who wants cheap accommodations I’d advise booking ahead, although the extent to which it’s necessary depends on what cities you are visiting.

 
Group of FOUR (age/gender: 50M, 28F, 18M, 10M)
I say the age for you to have an idea of what I’m looking for.
Were still not sure of all the cities, but here are some that are in the script:
Madrid, Barcelona
Paris
Rome
Athens
Istambul
Prague
Berlin
Amsterdam
London

You can book a 4 bed dorm, and then the 4 of you will have your own private room, and sometimes (depending on the place) your own private bathroom.

4 bed dorm in a hostel do you mean?

Hostels sometimes call these “family rooms” and in peak season, should be booked in advance.

I wasn’t clear if you were additional or included in the people you listed, but now I see you want four.
 
There are many more quad rooms available than rooms for five, so that part is good. But in many cases, your best bet is to get two hotel rooms. Often this will work out cheaper than chasing down an elusive quad.
 
For 20 nights, 10 major cities so widely spread out is not realistic.

 
You’d advise me to cut down to how many major cities then?

I’d advise 3-5 nights per major city, for those you have listed, and work out a reasonable progression between them so you aren’t backtracking nor spending too long on trains. You might want to include some budget flights along with trains. Athens will take a huge chunk of time and extra expense, and it’s not typically a #1 backpacker favorite (although the Greek Isles are), so if you want to start cutting I’d either start there, or I’d refocus the trip to include more of Greece. Istanbul is also out of the way, but you’d probably have time to do Greece and Turkey if you focused on those two countries.

Ok, so I’m gonna cut out Greece and Turkey and leave that to a separate trip another time.
 
So, back to what we were talking about, it is better to search for cheap hotels and get two different hotel rooms right? Can you give me a tip in how to do that??
 
Thank you alot!!!

These are most of the sites I use:
 
http://www.hrs.de
http://www.hotelclub.com
http://www.ratestogo.com
http://www.booking.com
http://www.kayak.com
http://www.gtahotels.com
http://www.octopustravel.com
http://www.bestwestern.com
http://www.accorhotels.com
http://www.hotels.de
http://www.venere.com
http://www.hostelworld.com
http://www.hostelz.com
http://www.travelodge.co.uk (for Ireland and the UK)
http://www.smoothhound.co.uk (B&Bs in the UK)
http://www.studios92.com (UK and elsewhere)
http://www.travelstay.com (UK mostly)
 
I also bid on Priceline and use Hotwire – but I refer religiously to www.betterbidding.com for advice. Bidding on Priceline is my favorite option for London, but you would need two rooms for sure.
 
Keep in mind that twin rooms have two single beds, but often these are adjacent to each other. Double rooms typically have one bed (which can be a smallish double) but they can also have two single beds adjacent to each other.
 
In a hostel dorm for 4 people, each of you would probably get an individual bunk bed. This might be the best option for space and privacy after all, if you don’t want to end up sleeping right next to each other, which is typical in European hotel rooms.
 
For four people, booking at this late date, you can expect rates of approximately 100 to 120 Euros per night for a 4-bed hostel room, and rates of 60-100 Euros per night per double/twin room. If you do better than that, consider it a bonus. Check reviews on TripAdvisor.com, BugEurope.com, and Hostelz.com .
 
If you do want a 4-bed hostel room, it’s probably best to contact the hostel directly to make arrangements to ensure you get what you want, and don’t end up in 4 separate dorm rooms.
 
If you are willing to stay a week in a place, you could probably get apartments, at least in some cities.

Hi JVBO,
 
A good alternative to hostels would be www.eurobookings.com (example for your trip would be this group of hotels in Spain: http://www.eurobooki…. The great thing about this site is that they have no reservation fees and no cancellation fees.
 
I wish you the best of luck with your trip!!
 

sorry about that wrong link – try this (for Spain): http://www.eurobookings.com/destinations-es/spain.html
 

Try Homelidays
 http://www.homelidays.com/EN-Holidays-Rental/100_Home/Home.asp?

=> lots of appartments and B&B´s, especially in France, Italy and Spain.