Hello mudshark, and welcome to the forum.
Right, where shall we start. Every camp site has it's big fans, and depending on who you ask, you will get a different answer. I'm sure if you have a look round the forum, you will find plenty of comments regarding the various sites. However, in a nutshell the most popular ones are Maison Blanche and Houx Annexe.
Sites such as Maison Blanche, Karting Nord and Terte Rouge are very close to the track, and it is possible to view the cars without moving very far from your tents. Sites such as Houx and Houx Annexe are a bit further, but have the advantage of being closer to the amenities in the Pit Village, such as the now infamous Poo Bar.
The camp sites, with one exception at the moment, are basically a free for all with regards to camping area. It is a case of first come first grab, so in order to make a camp for 15 to 20 guys you will need to get down to LM fairly early, but as I see you are planning on a week long trip, that should not be an issue. The exception is Houx Proper, which has marked and numbered pitches, and is favoured by myself for the reason my group don't go till late in the week, and it is a safe way to ensure a guaranteed area together. It is also the only site with "official" electrical hookups for the spaces, meaning no need for generators.
I think that all the sites now have finally managed to get reasonable shower and toilet facilities, although most of these are temporary so you cannot expect hot water all the time. Also as the camps fill up, it is not unusual to have long waiting times for said facilities, which leads to the dedicated 4 am shower crew (they are normally empty late at night).
To be honest, I would recommend that if at all possible, one or two of you try for a quick visit this year to have a look around. It is a perfect excuse for you to come on down
As for booking, you have several options. The first is to book directly with the ACO, who I believe start to take requests for the following year during the race. However this can be dodgy, because you don't always get allocated what you request. It is the cheapest method of getting tickets though. Next, you can get tickets from various agencies in the UK. Some of these also insist on doing an entire travel package (such as Motor Racing International) and others will sell race tickets only such as Just Tickets. This is a more expensive way of getting the tickets, but you will get what you book. Just Tickets have already started taking orders for 2006 and it is advisable to think about getting a booking in soon. MRI tend to issue their brochure in December, when there is a mad rush to get what they have. Finally there is the dreaded ebay
Personally I would not go near it with a barge pole, but in an emergency you can get tickets here, generally at very inflated prices.
I guess that others here will have a different opinion, but if I were you, I would look at booking your tickets now with a UK ticket agency such as Just Tickets. This way you can plan your trip safe in the knowledge you will have all the tickets you need. Also, I know it adds quite a lot to the cost, but I would consider getting Grandstand tickets as well. This way you get to see the start/finish action without having to be there hours ahead, and for a big group, makes for a decent meeting place. Also should it rain (heaven forbid) you do get some protection.
I hope this info helps, but ask away for anything you need to know. There are plenty of people willing to help on this forum.