You cannot go to the Tenancy tribunal because it is between two people and no house was involved.
"Only landlord-tenant disputes are heard by the Tenancy Tribunal. If there are any issues between tenants or flatmates, you can go to the Disputes Tribunal."
Yes i found out the tenancy tribunal is only for between tenants and landlords... they were my flatmate who had contract by flatmate agreement. Yes. I'm considering to take a dispute tribunal, but i don't know their present address now if i want to proceed it