Unique to the Cube engines, co-op edit lets you build and modify maps with others seamlessly. This "game mode" just allows you to switch to the edit mode online, just like as if you were editing locally. Some functions however are not available in co-op edit, namely the undo function, along with some convenient commands like gamespeed.
Saving/Resuming Co-op Sessions
Lighting, or saving, is done client side, so while still completely functional, it requires co-operation to keep everyone in the server up to date. This means if you do a calclight, everyone else will still be on fullbright or the previous calclight. Saving will just save a local copy of the current map to the hard disk drive, think of it as 'dumping' the map off the server, rather than saving. So if you're with some friends, urge them all to save the map occasionally so you don't lose your work.
When resuming a co-op edit session, simply use the command /sendmap and /getmap to send and receive the map through the server. Note that with this method, *.cfg files are not sent, meaning if you've got a custom texture list in the map's config, everyone else will see another random set of textures instead. If you're going to co-op edit with a custom config, you'll need to send the map config by other means to your friends co-op editing with you. People who join won't be able to see the map properly, so you should probably set the server to private (using
mastermode).
Editing Etiquette
Co-operative editing is just that - you can add to other's work or remove it. It is poor form to delete or "overwrite" other's work without their consent, just like it wasn't nice to destroy the other kid's castle in the sandbox. Co-op editors try to not work next to each other to avoid interference.
When choosing a server to co-op edit on with your mates, use your common sense. Server hosters despise co-op editors who join a 32 player server and set it to private. There are many specialised co-op edit servers which might have client limits of 6 or less, which is more than you'd want in a typical co-op edit anyway. As a general guide, servers with 8 player slots or above shouldn't be used for co-op edit, you could be stopping a group of players from having a game of capture, which they can't do on the smaller 4 to 6 player slot servers.
Learn to control your temper. If a newbie joins and starts messing with your map, don't lose it. Explain calmly and concisely why you don't want them to do whatever they are doing, and try to give them a few pointers. If you don't want outsiders to join, set the server to private (mastermode 3).
Co-op editing can speed up repetitive or boring processes especially for large and complicated maps, like smoothing lots of curves or building long stairways. Other editors can fix errors that you didn't notice with a method you didn't know about. It is important in coopedit to share editing techniques, so that even newbies can help you out. Think of editing Cube maps as writing wiki articles: anyone can contribute, and even if suffering some damage (inflicted deliberately or not) it can be easily reverted back to a normal state.