1.9 Solution

Place the following command in a repeating command block set to always active:

/execute @a[x=a,y=b,z=c,r=d] ~ ~ ~ tp @p e f g

Where:

• a, b, and c are the XYZ co-ordinates that you want to teleport players from
• d is the radius in blocks around that point where players will be selected (min 1)
• e, f, and g are the XYZ co-ordinates that you want to teleport players to (i.e. the destination)

1.8 Solution

Use the same command as above, but since no repeating command blocks exist, place the command block on a fill clock.

Put them in the Spawn Chunks

The Spawn Chunks are a 16×16 set of chunks centered around your world spawn. These chunks are loaded at all times, unless no player has been in the overworld in the last 60 seconds^*.

While some mechanisms are not processed when no player is around, such as plant growth or entities, redstone and command blocks are executed normally.

^{* According to the wiki. However, I have not been able to observe the unloading of the chunks on 1.8.7 in a singleplayer test world. IIRC SP is functionally identical to MP.}