The isboxer discord channel for classic has the following pinned message:

Looking for a multiboxer guild? Checkout the guilds on these servers
NA (USA...)
Name - Faction - Server - GM
Epic - Alliance- Blaumeux (PvP) - MooreaTV
Legion - Alliance - Herod (PvP) - Nodoze
Overload - Horde - Herod (PvP) - Zal
Cardboard - Horde - Benediction (PVP) - Blueblood


Europe
Name - Faction - Server - GM
Overkill - Horde - Zandalar Tribe (RP-PVP) - Stridsberg
Playing With Myself - Alliance - Nethergarde Keep (PvE) -Beeq
Never Alone - Alliance - Pyrewood Village (PvE) - Vivan