Most guilds have their own Discord for organizing end game raids. Some active Alliance guilds on US East Coast cluster (Ashkandi, Mankrik, Pagle, Westfall, and Windseeker) currently include: Encore, The Unyielding, The Misfits, Phoenix Rising and C ERA. Of course, you can talk to players in-game, who were pretty friendly, IMO.
Encore
https://discord.gg/psuuS5e8gn
The Misfits
https://discord.gg/h9avMaTYq5
The Unyielding
https://discord.gg/Vm6rwSTYpr
Phoenix Rising
https://discord.gg/rJHKR43ytS
C ERA
https://discord.gg/4q5ABRE6vD
There are also global Classic Era Discords as well:
WoW Classic Era (NA PvE East Coast) - see #raid-schedules Channel
https://discord.gg/NChc3Z4Bxs
Vanilla Classic Era - see #raiding Channel:
https://discord.gg/Nnv8qgJpC3
PUGLORD’S R US
https://discord.gg/VqvnBwCvBd
WoW Classic Era [PvP]
https://discord.gg/WQ7ZfcbRP4
Also see:
https://www.reddit.com/r/classicwow/comments/vy59yd/undead_era_the_actual_state_of_the_classic_era/
Hope that helps.