I’d think BEAT is your best bet. I met a few people from BEAT and they seemed great, I just lived in Nashville proper and the workouts were a little far away for me.
Also a BEAT member, great group of people. Great training variety, terrain and abilities around here, take your pick!
There are a number of swimming options around, though maybe none more convenient than Maryland Farms Y if you live right in/near Brentwood. Williamson County Indoor Sports Complex has a group also, not sure how strong they are now though. If you wind up closer to town or don’t mind a drive, there’s a big group of masters & triathletes that swims up at the Centennial Sportsplex as well. Open waters on Sunday, too.
I think there’s a Nashville Triathlon Club as well, not really positive. Also many training groups associated with different areas or businesses. Great group rides from a shop named Trace Bikes over in an area called Bellevue. It’s a bit of a haul but a good group of people. I drive over for that ride once or twice a week, usually. Also Harpeth Bike Club has good weekly rides, as does RB’s, which is moving to right in the Cool Springs area.
Tons of great options here, good people too. I’ve been here about 3.5yrs now & love it.