db.users.aggregate([{$group: {_id:"$emails.address"}}])

{ "_id" : [ "a@a.com" ] }
{ "_id" : [ "b@a.com" ] }
{ "_id" : [ "c@a.com" ] }

["a@a.com","b@a.com","c@a.com"]

{
    "_id" : "ukn9MLo3hRYEpCCty",
    "createdAt" : ISODate("2015-10-24T03:52:11.960Z"),
    "emails" : [
        {
            "address" : "a@a.com",
            "verified" : false
        }
    ]
}
{
    "_id" : "5SXRXraariyhRQACe",
    "createdAt" : ISODate("2015-10-24T03:52:12.093Z"),
    "emails" : [
        {
            "address" : "b@a.com",
            "verified" : false
        }
    ]
}
{
    "_id" : "WMHWxeymY4ATWLXjz",
    "createdAt" : ISODate("2015-10-24T03:52:12.237Z"),
    "emails" : [
        {
            "address" : "c@a.com",
            "verified" : false
        }
    ]
}