question

>> When they join a multiplayer game (servers are hosted via Photon, not PlayFab) all other players need to be made aware of what items are currently active. What's the best way to do this?

Do you mean you want to let players see each other’s items, such as some equipment? If so, we would suggest you use Play Read-Only Data to store the corresponding items’ info. Then let players call Client API GetUserReadOnlyData to read other players’ item lists. Clients needn’t send the JSON to servers directly.

You can write a CloudScript function to verify whether the items they want to equip is in their Inventories. If the inventory contains the items, CloudScript can store the item lists to Player Read-Only Data. You can call server API GetUserInventory on CloudScript to get the player’s inventory. If you want to learn about how to set the Player Read-Only Data on CloudScript, you can check this documentation How to modify read-only or internal player data from CloudScript.

>> Is there some API that the clients can call to get the currently worn items for other player IDs without needing an admin API key perhaps?

As the first answer said, clients can call Client API GetUserReadOnlyData to read other players’ read-only data. If you want to let the specific K/V pair can be read by other players, please set the permission of the K/V pair as “Public”.