Minecraft server spawn command

Solution 1:

Assuming I understand your question correctly, you want to change your worlds spawnpoint. Doing this is easy, what you need to do is go to where you want your spawnpoint to be and while standing at the designated place you've decided on, type;

/setworldspawn

It may be helpfull to first convert your world to creative mode so you can fly around and find a suitable location easier, then, before you invite your friends change your world back to survival mode.

/gamemode survival
/gamemode creative

Solution 2:

If you are using vanilla Minecraft server, /setworldspawn will do the job perfectly. If you are on a CraftBukkit server, try /setspawn or /gm 0 for survival and /gm 1 for creative.