A DAML model showcasing unbounded signatories https://medium.com/daml-driven/removing-the-limits-on-signatories-6f947ab19361

Celebration.daml

-- Copyright (c) 2019, Digital Asset (Switzerland) GmbH and/or its affiliates.
-- All rights reserved.

daml 1.2
module Celebration where

import DA.Date qualified as Date
import DA.List

template Celebration with
    participants : [Party]
    date : Date
    occassion : Text
  where
    signatory participants

template CelebrationProposal with
    confirmedParticipants : [Party]
    pendingParticipants : [Party]
    date : Date
    occassion : Text
  where
    signatory confirmedParticipants
    ensure not (null pendingParticipants)

    controller head pendingParticipants can
      Confirm : ContractId CelebrationProposal
        do
          assert (length pendingParticipants > 1)
          create this with
            confirmedParticipants = head pendingParticipants :: confirmedParticipants
            pendingParticipants = tail pendingParticipants

      ConfirmLast : ContractId Celebration
        do
          assert (length pendingParticipants == 1)
          create Celebration with
            participants = reverse (head pendingParticipants :: confirmedParticipants)
            date
            occassion

test = scenario do
  [alice, bob, charlie] <- mapA getParty ["Alice", "Bob", "Charlie"]
  let date = Date.date 4 Date.Apr 2019
  prop1 <- submit alice do
    create CelebrationProposal with
      confirmedParticipants = [alice]
      pendingParticipants = [bob, charlie]
      date
      occassion = "DAML is openly available"

  prop2 <- submit bob do
    exercise prop1 Confirm

  celebration <- submit charlie do
    exercise prop2 ConfirmLast

  pure ()

In Confirm you should assert length pending Particpants isn't 1, otherwise you can confirm and create a contract with zero pending which is stuck. Alternative is just have one confirm, and have it do different things based on number left.