Distance between Talllinn and Viljandi is ca 160 km; Riga and Viljandi 224 km (if you arrive through Riga then you need to drive by bus to Pärnu, then from Pärnu to Viljandi).
If you arrive by plane then it is possible to get from the airport to the Tallinn bus station either by city bus or tram (distance is about 10 minutes), the bus/tram stop is named "Bussijaam" in Estonian (airport is named "Lennujaam" in Estonian). Please see the further information: https://www.tallinn-airport.ee/en/transport/leaving-the-airport/
If you arrive by ferry to the Tallinn port then take a bus no 2 to the Tallinn bus station ("Bussijaam"): http://www.portoftallinn.com/transport-parking
Long distance bus schedules (for example from Tallinn to Viljandi; or from Riga to Pärnu, then Pärnu to Viljandi): https://www.tpilet.ee/en
It is also possible to travel between Tallinn and Viljandi by train (in Viljandi you need a taxi to get to the central town): https://elron.ee/en/
If you need a taxi in Viljandi, call +372 1200 or +372 1300 or +372 1700 (the longest distance in VIljandi is from railway station to central town, other routes are short and walkable).
The participants of the conference were accommodated in Grand Hotel Viljandi which is about 10 minutes walk away from the Traditional Music Centre. Every participant was guaranteed to have one bed.
Grand Hotel http://www.ghv.ee/en