I am not sure if you know how to script, or how comfortable you are if you do. This is an overview of a method of doing what you want.
If you look at the
list of scripting commands, you will notice there are two commands,
onPlayerConnected and
onPlayerDisconnected. These allow you to execute code anytime a player connects or disconnects. You can also use
getPlayerUID to get the ID of the player and store their position and ID in an array when the leave, and set it when they re-join.
The issue that I see is that if the mission restarts you lose the data. Therefore, if you restart the mission you cannot move the player back to their previous location since it is now gone. A way around this is to extend ArmA II with a mod that either rights to a file, or better yet connects to a database.