The new system coming in Book 8 for the new instances assigns 1 token to each player in the party after successfully completing challenge mode. You must loot the chest to receive the token. Similar to how a quest item drop works. Depending on the instance you will need 4 or 7 tokens to trade for the radiance item.
The older instances will continue to use the current method of distributing 1 barter item per successful challenge mode completion. We are looking at changing this to match the new Book 8 mechanic (taking away a large portion of the grind for most players) but it won't make it into Book 8.