Well, here's a simple step by step guide for OpenCanvas, hoping it'll be useful for the people wanting to take the first steps
First of all, you'll need the program itself: You can download it here:

http://files.filefront.com/oC11b72exe/;5433314;/fileinfo.html

It's not that easy to find it online (with google you most likely will find the full non-network and expensive version), since it has been discontinued from version 1 and it's free.
The OC Program is just a simple .exe file, no need to install anything, just keep it on your desktop for easy access! Here's the exact name of the EXE file you'll be downloading:



Now Double click on the .exe file. If this is your first time using Open Canvas, it will open up in japanese language and most of the things will look rubbish to you. Roughly, you see the menu bar at the top, the preview panel in the top left corner, the tools selection menu right under it, the layers panel in the lower left corner, and the brush color picker-mode and settings in the lower right corner.



Here's what you need to do: Click on the rightmost menu in the top menu bar and choose "English":



Once done this, most (but not all) functions will be written in English. At this point you can start doodling by just going into File-New and selecting a size for your sheet but let's go see the network thing.
To start a network drawing session (or join into an existing one) you need the network window. You can find it under the "Network" menu: Just click on it and choose "Show Network Window".



Done this, two new windows will appear in the center of the screen: the Chat window (now unusable and empty" and the Network Window. In this last one you have three tabs.



The first tab, server, is to make your computer into an OC server. You just have to set the name of your server, the optional password, and click on "stand by". Other people will be able to connect to you this way.The last, Network Preference, lets you choose the number of people that can join (max 4 if you use your OC program to be server), the number of layers allowed to each person, the size of the sheet and the network port to use for communication (this must be port forwarded in your router and allowed by any firewall if you want people to be connected to you).
The option that interests us, though, is the second one, since we're joining an ever-running OC server maintained by Empe. So without further hesitation, click on the "Client" tab. Here's what will happen:



In this one you just need to type the info necessary to connect to the OC server: IP address (of the server, so you must know this beforehand), your nickname (you can choose whatever you want of course), and the password required for the server (if any is).
Important: If you don't know the info for LTS's OC server, ask them to me via private messaging!
Once typed in this info, and if all is correct, click on the button on the right and you will be in the game!



The server is set up to allow up to 24 people connected and up to 24 layers usable. It has playback function, so if you enter and people has already started to draw, everyone should stay still and give OC the time to "redraw" the drawing on the screen of the newcomer. But it's not mandatory, it's just to make life easier and avoid having to redraw everything everytime.
When you close the window, a prompt comes up asking if you want to save the session. If you say yes, this creates a .wpe file that is basically a "movie" of what has been done, and can be replayed anytime in OpenCanvas (just drag the .wpe file over the opencanvas executable).

Last but not least, the OpenCanvas chat window does not work as intended, but you can use it to type some commands explained earlier in the thread, like "/who" to see who is connected to the OC server. It's best to join LTS chat when OC is joined, so to keep the artists coordinated

Thanks for writing this, Marco! (read: idiot-proofing it for me ).

I haven't tried connecting yet, but after my test doodle I have a couple of technical questions:

1) Is there any way to change the opacity of whichever brush you're working with?

2) As you can see from the test pic, the top was lopped off when I tried to change the canvas size, and there's no 'undo' fuction for that action. Is there any way to get around this?

3) Failing that, is there any sort of selection tool?

That's very weird. I would have thought they'd be something to do with R/G/B colour values... Thanks for figuring it out; I'll give that a go. 

Test the Second (I may or may not be connected; not sure).

Making things opaque is hard.

*Note:

OC Server going offline for about 20 mins for server upgrade, will be back shortly!

OK Server now upgraded, now has 3Gb memory installed as all the server stuff was starting to impact ram (not so much OC server but other stuff) hopefully shpuld still be runnong ok. pleae let me know if any problems!