There are server clusters now:
I think there are a couple of raid groups.
I know that the Anathema, Arcanite Reaper, Bigglesworth, Blaumeux, Fairbanks, Kurinnaxx, Rattlegore, Smolderweb, Thunderfury, and Whitemane cluster has an Alliance raid group, and I think also a Horde raid group.
There was also a Horde guild that I think raids on Earthfury, Herod, Kirtonos, Kromcrush, Skeram, Stalagg, Sulfuras, and Thalnos, iirc it’s called <Classic Forever>.
I’d recommend logging into a new character, and checking LookingForGroup (world doesn’t seem to have as many active people), and ask around to see what activity you can find.
Also, maybe encourage people who are still playing Classic Era to post recruitment threads on the forums from time to time.