how to find most common element from list

You can create a simple SKILL function to accomplish this, for example:

procedure( findMostFrequentUser(listOfUsers "l")
  let((userTable (maxCount 0) maxUser)
    ;; Count the user occurrences
    userTable = makeTable('userTable 0)
    foreach(user listOfUsers
      userTable[user] += 1
    ); foreach
    ;; Get the maxUser
    foreach(user userTable
      when(userTable[user] > maxCount
        maxUser = user
        maxCount = userTable[user]
      ); when
    ); foreach
    maxUser
  ); let
); procedure

users = '("user1" "user2" "user3" "user1" "user1")
printf("Max user: %L\n" findMostFrequentUser(users))

Running the above gives the following output:

Max user: "user1"