With this command block setup in your server world, all players will get achievements when anyone gets them. This lets you cooperate on getting all the achievements, instead of each player having to get them all on their own.

If a new player joins in, he or she will immediately get the achievements of the players already on the server.

To get it, download the schematic and use MCEdit to import it into your world, or paste this command into a command block and trigger it with a button (has to be a button, not a redstone block).

The device will install itself automatically and start running immediately.

Note: This has now been updated to fix the problem that plagues most of these systems: that you can skip achievements. If you want to know the details, check out this video.