break out parseInput - it's good to have dedicated parse stage indicating parsed type.

GroupExample.hs

import Data.Function (on)
import Data.List (groupBy)
import Data.Maybe (mapMaybe)
import qualified Data.List.Utils as U

type Domain = String
type NameServer = String

main :: IO ()
main = interact process
  where 
    process :: String -> String
    process = unlines . map show . parseInput

parseInput :: String -> [(Domain, [NameServer])]
parseInput input = map squish groupedPairs
  where
    groupedPairs = groupBy ((==) `on` fst) pairs
    pairs = mapMaybe parseLine (lines input)

parseLine :: String -> Maybe (Domain, NameServer)
parseLine line = case U.split "ORG" line of
                    (d:n:[]) -> Just (d, n)
                    _        -> Nothing

-- create (domain, [ns1, ns2, ns3]) tuples
squish :: [(Domain, NameServer)] -> (Domain, [NameServer])
squish [] = error "BUG: empty domain group given to squish!"
squish full@((d,ns1) : _) = (d, map snd full)