Database
db={"members":[{memberId:1,name:"william",gmail:"william@gmail.com"},{memberId:2,name:"Alfred",gmail:"afd@gmail.com"}],"groups":[{name:"Dairy",members:[{memberId:1},{memberId:4}]},{name:"Grocery",members:[{memberId:1},{memberId:2}]}]}
Query
db.groups.aggregate([{"$lookup":{"from":"members","localField":"members.memberId","foreignField":"memberId","as":"members"}}])