[
 {
  name:"member1",
  activity:"activity1",
  selected:true
 },
 {
  name:"member1",
  activity: "activity3",
  selected:false
 },
 {
  name:"member2",
  activity:"activity2",
  selected:true
 },
 {
  name:"member2",
  activity: "activity4",
  selected:false
 }

]

[
     {
      name:"member1",
      activities:[true,null,false,null]
     },
     {
      name:"member2",
      activities:[null,true,null,false]
     }
    ]

db.collection("MemberActivities").aggregate(
    [
        {
        $group:
            {
            _id: "$MemberName",
            activities: { $push: "$selected"}
            }
        }
    ]

[
     {
      _id:"member1",
      activities:[true,false]
     },
     {
      _id:"member2",
      activities:[true,false]
     } ]