Quoted from my answer to How do workers work when I gain control of them?:
When you get control of someone's
units because they've left the game,
you can command their units and buildings as you like (attack, lift off, change rally points, use energy, etc), and you
can also spend minerals and gas. You
can build buildings, train troops,
etc. The resources come out of your
bank, but this is because when the
ally leaves, their resources are split
among remaining allies, and their
further income gets split among you
Although you are spending "your"
resources, the buildings and troops
remain that player's for gameplay
purposes. Pylons will make energy
fields for them, not you. Depots will
raise their supply cap, not yours (and
they can be supply capped even if
- If an ally drops early, make sure to send their workers harvesting, and train more. It will cost you some minerals, but you will get lots back from the split harvest.
- If an ally drops late, you can basically take over for them, building bases and troops and waging war. At the very least, you can add their remaining troops to your army.