The command is /spawnpoint @a[coords] but it won't affect the multiplayer spawn point. You need a program like NBTExplorer to set the actual map spawn coords. Also, I have noticed a bug that does not allow players to spawn at the correct spot in multiplayer. It usually puts them at a random spot close to the spawn point that is set... weird. Hope this helps.
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You can use this for multiplayer spawnpoints if you want a centain spawnpoint for a start of a map or a server. The contraption needs to be placed in the maps spawn chunks otherwise the clock could brake upon restarting the map. The only thing you need to adjust are the coordinates for your spawnpoint.