[{_id:"1",companyId:"xyz",lastModifiedByUserEmail:"name@domain.com"},{_id:"2",companyId:"abc",lastModifiedByUserEmail:"name@domain.com"},{_id:"3",companyId:"abc",lastModifiedByUserEmail:"name@domain.com"},{_id:"4",companyId:"xyz",lastModifiedByUserEmail:"name@domain.com"},{_id:"5",companyId:"xyz",lastModifiedByUserEmail:"bob@domain.com"},{_id:"6",companyId:"xyz",lastModifiedByUserEmail:"bob@domain.com"},{_id:"7",companyId:"xyz",lastModifiedByUserEmail:"bob@domain.com"}]

db.collection.aggregate([{$group:{_id:{"lastModifiedByUserEmail":"$lastModifiedByUserEmail","company":"$companyId"},total:{$sum:1}}},{$sort:{total:-1}},{$limit:1}])