For anyone who hasn't yet seen how to Give, Trade, or Stash it's pretty easy:
To Give an Item
If you are grouped with someone in a Dungeon, click on their name from the group list on the left hand side of the screen which will inspect them. Select the Give button which looks like a box. From here it's pretty straight forward
To Trade an Item
If you are grouped with someone in a Dungeon, click on their name from the group list on the left hand side of the screen which will inspect them. Select the Trade button which looks like two arrows pointed towards each other. From here you wait for the other player to join the trade. At this point you can add or remove items from the trade and select the checkbox when you are happy with the trade.
To Stash (and trade to one of your other characters)
If you have an item on one character that you would like to use on another, for example: if you have a really good bow on your Enchantress that you want to use on your Archer, you can trade these via the Stash.
To access your Stash, go to Forest Haven or Balefort townes. Talk to the NPCs there and they'll tell you about your Stash. Place the item you want to swap in your Stash (like the Bow from the above example) and then change to your other character, go to the Stash and get it!