Monte argentario

This information source is reliable and recent: https://www.wanderingitaly.com/maps/monte-argentario.html

You will need a car unless you plan to just chill on the promontory. Orbetello is the closest train station, and you may be able to rent a car there. Bus would get you to the MA villages, but I don't recommend that, again unless you plan to perch on a seaside and stay without day trips. Orbetello has the most shops/restaurants, but the two villages on MA have restaurants, and there are some up in the hills as well. We stayed in Orbetello--apartments are limited on MA but you might find something.
The promontory is very rocky (summer would be very hot), but there are sandy beaches on the spits connecting to the mainland. You can also take a ferry to Giglio.
We spent our time in the Ucellina nature park, visited Pitigliano and Saturnia hot springs. You might also look at staying between Massa Marittima and Castiglione della Pescaia.
What time of year?

We will be there mid June.

It will be hot. Last year, there was some kind of insect infestation in the area, which I believe primarily affected Orbetello. I would google to see when and where the affect was. That would not necessarily keep me away, but I would investigate. It is one of my favorite areas after seven visits to Italy, and the Etruscan heritage and scenery are unique.

Thank you so much for this information. The hot springs sound wonderful. Do you think it would still be worth the visit during mid June?

Do you mean for the hot springs or the area in general?
As long as you are okay with the heat, should be great. I found Saturnia too crowded to enjoy, and I visited in May. But I went in late afternoon, which is likely the most crowded time of day.

I was wondering if you thought it would be too hot for the springs in June. We will be there June 14-19 and I imagine it will be getting warm in general by that time.

Not sure--I believe it is a year-round destination, though true, unlikely to be appealing in the heat--I'd choose a beach instead, you are spoilt for choice!

I'm very familiar with the area.
Coming from Sinalunga, you need a car.
You also need a car to properly visit the entire Maremma area, where Mt. Argentario is.
I like Porto Santo Stefano and also Porto Garibaldi at the Monte Argentario. From P. S. Stefano you can take a ferry to Giglio island (45 min). Last time (2018) I stayed at this hotel in Porto Santo Stefano.
Saturnia and its springs is a great rural area. I went there last time this past July 2022. It was hot for sure, but the water at the Springs is not super hot (just warm), so it is not unbearable. I used to go there a lot in winter and in winter the water feels hotter since out of the water is so cold and you feel the difference. The famous free hot springs at the Cascate del Mulino (watermill falls) was packed in July in spite of the 100 F + degree weather.
Saturnia has a famous resort hotel called Terme di Saturnia. I stayed there a long time ago, but in 2022 I stayed at this hotel just 5 min away but much cheaper (they also have discounts if you want to use the Terme di Saturnia hotel.
If you have a large group, maybe a villa or agriturismo is cheaper, but of course you wouldn't have the services like massage, hydrotherapy and all that stuff.

Maremma, in southern Tuscany, IMO, is one of the most beautiful areas of Italy, so I recommend it. But a rental car (or two for your large group) is an absolute MUST. If you have at least two adults willing and able to drive, rent two small/rental cars for 6 people. Do not rent a van because one van costs as much as 3 cars to rent.

Roberto - thank you do much for this helpful information!!