{
    "mlVoters" : [
        {"email" : "a@b.com", "isApproved" : false}
    ],
    "egVoters" : [
        {"email" : "a@b.com", "isApproved" : false},
        {"email" : "c@d.com", "isApproved" : true}
    ]
},{
    "mlVoters" : [
        {"email" : "a@b.com", "isApproved" : false},
        {"email" : "e@f.com", "isApproved" : true}
    ],
    "egVoters" : [
        {"email" : "e@f.com", "isApproved" : true}
    ]
}

db.documents.aggregate([
  {$project: { mlVoters: 1 } },
  {$unwind: "$mlVoters" },
  {$group: { _id: "$mlVoters.email", mlCount: { $sum: 1 } }},
  {$project: { _id: 0, email: "$_id", mlCount: 1 } },
  {$sort: { mlCount: -1 } }
])

{"mlCount" : 2.0,"email" : "a@b.com"}
{"mlCount" : 1.0,"email" : "e@f.com"}

{"egCount" : 1.0,"email" : "a@b.com"}
{"egCount" : 1.0,"email" : "c@d.com"}
{"egCount" : 1.0,"email" : "e@f.com"}

{"email" : "a@b.com", "mlCount" : 2, "egCount" : 1, "totalCount":3}
{"email" : "e@f.com", "mlCount" : 1, "egCount" : 1, "totalCount":2}
{"email" : "c@d.com", "mlCount" : 0, "egCount" : 1, "totalCount":1}