Press the use key on a moveable object to use Telekinesis on it.
The more PSI you have the heavier the objects you can lift e.g. cars.
Also, carrying around a friendly sentry this way can be neat. The sentry will still work, shooting enemies, while also working as cover for the carrying player. Also, sentries have unlimited ammo, making this a nice trick when having to deal with wave after wave of enemies.