IOUState:
@BelongsToContract(IOUContract::class)
data class IOUState(val value: Int,
                    val lender: Party,
                    val borrower: Party,
                    val thirdParty: Party,
                    override val linearId: UniqueIdentifier = UniqueIdentifier()):
        LinearState, QueryableState {
    /** The public keys of the involved parties. */
    //override val participants: MutableList<AbstractParty> get() = mutableListOf(lender, borrower)
    override val participants = mutableListOf(lender, borrower)


ExampleFlow:
var iouState = IOUState(iouValue, serviceHub.myInfo.legalIdentities.first(), otherParty, thirdParty)

iouState.participants.add(thirdParty)

val txCommand = Command(IOUContract.Commands.Create(), iouState.participants.map { it.owningKey })

val counterparties = iouState.participants.map { it as Party }.filter { it.owningKey != ourIdentity.owningKey }.toSet()

counterparties.forEach { p -> flowSessions.add(initiateFlow(p))}

val fullySignedTx = subFlow(CollectSignaturesFlow(partSignedTx, flowSessions, GATHERING_SIGS.childProgressTracker()))

            // Stage 5.
            progressTracker.currentStep = FINALISING_TRANSACTION
            // Notarise and record the transaction in both parties' vaults.
            return subFlow(FinalityFlow(fullySignedTx, flowSessions, FINALISING_TRANSACTION.childProgressTracker()))