Train
Trains between major cities will have non-stop, high speed options. The main train station in Florence is Firenze Santa Maria Novella (SMN). Once in Florence, you would take a regional train to Poggibonsi-S.Gimignano. Regional train is 1 hour and around $10 per person. Once in Poggibonsi, you can call local taxi: +39 0577 937907 or ask the station bar for help. You can also try https://ittaxi.it.
Flight
Feel free to explore Italy before or after the wedding! There are high speed trains between major cities in Italy so you do not have to fly into Florence, although it is the closest airport to Poggibonsi.