[PvK]

Unless the game options are set to disallow trading, you can give ships and units and resources to other players.

They can automatically resupply at your planets if you give them a certain treaty level or higher.

Literally picking up items off your planet, not per se. To transfer units, you need to trade them a ship or planet that holds the units. I've sometimes build a ship or base which is just a simple container to give an ally some units, but I haven't had occasion to do that often.

PvK