[{"user":"a","domain":"some.com"},{"user":"b","domain":"some.com"},{"user":"b1","domain":"some.com"},{"user":"c","domain":"test.com"},{"user":"d","domain":"work.com"},{"user":"aaa","domain":"work.com"},{"user":"some user","domain":"work.com"}]

db.collection.aggregate([{$group:{_id:"$domain",data:{$push:"$$ROOT"}}},{$addFields:{data:{$slice:["$data",0,2]}}},{$unwind:"$data"},{$replaceRoot:{newRoot:"$data"}}])