I don’t know about Moonguard but if Sargeras (very high pop alli realm) is any interest to you then Bloodbound is always welcoming people. We’ve long been the #1 PvP guild on the server and among the top RBG guilds in NA. There are some other PvP and/or WPvP guilds too but not as large or active. Could make a level 1 toon on Sargeras and check out trade chat to see if it is active enough for you.