[{_id:ObjectId("6087feeef467a4320883daf3"),name:"group 1",admins:["adam","ken"]},{_id:ObjectId("2087feeef467a4320883daf3"),name:"group 2",admins:["rebecca"]},{_id:ObjectId("9987feeef467a4320883daf3"),name:"group 3",admins:[]}]

db.collection.aggregate([{$unwind:"$admins"},{$group:{_id:null,adminsSet:{$addToSet:"$admins"}}}])