This can be done by completing the following:
- Invite the player to your party.
- Navigate to the player's name in the bottom left portion of the menu.
- Choose the "Invite to Trade" option.
- This will bring up the trade menu where you're able to see what items are available for trading, what you're willing to trade along with the other member.
- A request must be made by one player and the other player must accept the request for a successful trade.
Note: This cannot be done with a split screen party member.
Here is a video as a source:
