OK, so on my friends and my server, we have a skyblock map. My issue is when somebody joins they spawn above the Void. What I need is for a command block to determine if a player is near the Void and send a redstone signal to another command block that will teleport them to the main island.
You can make a redstone clock (I recommend the repeater ones) and attach it to a command block with this command:
/spawnpoint @a X Y Z
Remember to turn off command block output to prevent spam!
/gamerule doCommandBlockOutput false
Hope it helped. :)
What you need is if they "spawn" above void then go to the spot you want to be the spawn point and type in the chat bar /setworldspawn
This will set the worlds spawn to that spot instead of trying to have lots of command blocks
Why not just get the Essentials plugin and set the spawn somewhere else? You can use the /setspawn command. This is, of course, assuming you're using Bukkit.
Actually, you can just use
/setworldspawn and that will make everyone spawn in the same general area.