Database
db={"conversations":[{members:["124578987898787845658574","124578986532124578986532"]}],"users":[{"_id":ObjectId("124578987898787845658574"),"name":"foo"},{"_id":ObjectId("124578986532124578986532"),"name":"bar"}]}
Query
db.conversations.aggregate([{$addFields:{members:{$map:{input:"$members",in:{$toObjectId:"$$this"}}}}},{"$lookup":{"from":"users","localField":"members","foreignField":"_id","as":"members"}}])