Can I give a portal key to another player in Ingress?

There's no way to securely trade items, but you can transfer them from one person to another by dropping them and having the other person pick it up. Obviously, this requires both of you to be in the same place.

Capsules are commonly used to transfer larger numbers of items in this way.

Yes you may by dropping the item onto the floor. The other player would need to pick it up from the same place. There is an expiration date (~24 hours) for items placed on the ground and it will disappear for good if not picked up.

You can drop any other items on the floor for other players to pick up. For larger amounts, I'd recommend using a capsule to drop up to 100 items to another player. You can get a capsule (Rare) from hacking.

You can not give the key to a specific person, but you can drop it at a physical location and arrange to have them pick it up.