I've never been to Castelmuzio - it is kind of in the middle of nowhere by itself but the local area is full of the Tuscany destination small towns. You could probably put together a complicated train+bus transfer set up to get there but I think realistically a car is the most straightforward answer and it is a great area to have a car with short distances to a bunch of scenic places.
Siena has local train service to Sinalunga as well as Buonconvento and Sinalunga has 25 minute bus service to Castelmuzio. But a train to Siena to a train to Sinalunga to bus to your destination sounds like little fun. If you are coming into Rome you could train from Rome to Sinalunga (change in Chiusi) and bus/taxi/shuttle to Castelmuzio. Assuming this is a wedding venue I would check with them for recommendations or transfers they might provide.
You don't say much about your schedule but if you're going to be tied up with wedding things for several days in Castelmuzio then this might be fine. But if you're not it would be best to have some mobility in this amazing area of Tuscany. Just name a couple of the famous destination towns in this area - Pienza is 15 minutes away, Montepulciano and Montalcino are 40 minutes away.
Many people train to Chiusi - technically Chiusi-Chianciano Terme train station - and rent a car there because it is already close to this area versus driving from Rome or another big city. This is a great area to explore with small towns with a car.
The usual car rental caveats:
- Every driver needs an IDP before leaving the states.
- Read up on ZTLs in general and in specific areas before you go there.
- Add at least 25% to Google travel times for rural travel and expect to get lost.
That my quick overview - if you hesitate to rent a car checklist with the venue but this is a great area to explore.
Have a great trip!
=Tod