question

>> 1) Does 'MembersToMatchWith' require just the entity or the entity plus the attributes of other players?

As the API references mentioned, it requires just the entity of other players.

>> 2) Is this example of an entity complete for the purposes of MembersToMatchWith assuming attributes are not required?
{"Entity":{"Id":"538209432809432", Type":"title_player_account","TypeString":"title_player_account"}

“Entity:” isnot needed,the item of array MembersToMatchWith should be {"Id":"[YourTitlePlayerId]", Type":"title_player_account"}. You only need to use the field’s name listed in the API references.

You can refer to the following request body of CreateMatchmakingTicket.

{
 "Creator": {
 "Entity": {
 "Id": "[CreatorTitlePlayerId]",
 "Type": "title_player_account"
 }
 },
 "MembersToMatchWith": [
 {
 "Id": "[MemberTitlePlayerId]",
 "Type": "title_player_account"
 }
 ],
 "GiveUpAfterSeconds": 100,
 "QueueName": "[YourQueueName]"
}

>> 3) Given the entity above how would I turn this value from a string into a valid Json (Object?) that can be added to the MembersToMatchWith array in UE4 blueprints?

You can use “Set String Field” function that provided by PlayFab Plugin. Please refer to the following screenshot of UE4 blueprints.

>> 4) How would I add this object to the JSon array?

You can use UE4 native function “Add(Add item to array)” to add it to the Array. Please refer to the following screenshot of UE4 blueprints.