Database
[{"members":["userIdOne"]},{"members":["userIdTwo"]},{"members":["userIdOne","userIdTwo"]},{"members":["userIdOne","userIdTwo","userIdThere"]}]
Query
db.collection.aggregate([{$match:{$expr:{$eq:[{$setEquals:["$members",["userIdOne","userIdTwo"]]},true]}}},{$count:"count"}])