Meeting in Siena

You could meet their train. Or wait at the Duomo. Or at a cafe on the piazza where the Palio is run.

Do you mean this Saturday?

Since you'll have "several hours" to kill, consider storing your luggage at the bus station so that you can explore Siena without having to drag it around. Retrieve it later and take an inexpensive city bus down to the train station to meet your friends. I understand there's also now an escalator you can take down to the shopping mall across from the train station, but I'm not familiar with the logistics of that option.

Not sure there is a luggage storage facility at the bus stop on Piazza Gramsci.
In any case meeting your friends at the station is best. The rental car offices are down the hill too, not too far from the station.

When I last visited Siena a couple of years ago, there was a luggage storage facility in the underground section near the ticket windows. Maybe a more recent visitor can confirm whether it's still operating.