P:
All Players.
C:
Always.
A:
Modify score for Current player: Set to 100 Custom.
Disable use of computer players in leaderboard calculations.
Show Leader Board for most Custom. Display label: 'Lives'
P:
Player 1-6 (Assuming there's 6 players; change as necessary)
C:
Current player commands at most 0 men.
Current player Custom score is at least 1.
A:
Create 1 Zergling (or whatever the player's unit is) at Location X for Current player.
Modify score for Current player: Subtract 1 Custom.
Preserve trigger.
That's it. Since the place you want them to spawn will probably change everytime a different part is beaten, you'll need to take that into account. You could do that by changing the last trigger I wrote to this:
P:
Player 1-6 (Assuming there's 6 players; change as necessary)
C:
Current player commands at most 0 men.
Current player Custom score is at least 1.
Player 8 (whichever player is the Computer) has suffered exactly 0 deaths of Psi Emitter (or any other unused unit).
A:
Create 1 Zergling (or whatever the player's unit is) at Location 0 for Current player.
Modify score for Current player: Subtract 1 Custom.
Preserve trigger.
Then when they advance to the next area, make it so Player 8's death count for Psi Emitter is set to 1. Copy that last trigger and change the copy to "Player 8 has suffered exactly 1 deaths of Psi Emitter" and make the spawn location "Location 1". When they beat that area, have it so Player 8's death count for Psi Emitter is set to 2. Copy the above trigger again and change it to "exactly 2 deaths" and "Location 2". Repeat this process until you've set it up for every stage of the bound.
If you have any more questions, just ask