Yeah, it unfortunately comes with the hobby. It´s like groups for DnD, you probably wouldn´t find one that has been going on for a decade that is still searching for new people since at that point, they really don´t need them.
Now, there are guilds for casual RP (speaking stuff such as RPing civilians in main hubs, visiting markets or running a market stall, that sort of stuff), most often right within those hubs. If we´re talking Horde side, as you mentioned the Mag´har, you might be able to find something either in weekly market event place in Dawn´s Blossom in Pandaria, they´ve got market going there every Tuesday.
Alliance has more options, as Horde RP is less hub-oriented and more focused on guilds, which however won´t be that much of a problem since you are after a guild. There´s a Horde discord, although I´m not in it so can´t send you a link. From what I´ve heard, it´s the most likely place where you will be able to meet plenty of people from Horde scene and see if there is a guild that you might fancy.
Now, for RP-PvP, there´s Warsworn Discord:
The forum thread is locked, but the link there should work and the Discord is quite active.
However, there´s a disclaimer I have to put here, and that goes for RP-PvP specifically and RP in general, as there´s a very common misconception people outside of RP or new to it have:
RP doesn´t mean playing the game while acting as if you are a person in Azeroth.
Now, there´s nothing wrong with it nor people who RP that way, however it´s not something that RPers on this server mean when they use the word “roleplaying”. Rather, RP on this server is about creating a character, their story and interacting with other people/characters almost entirely outside of the scope of WoW´s gameplay.
It is important to understand that when you´re joining RP guild or a community, you´re not joining group of people who log in to the game and go run a dungeon or form arena team together. Some people within the guild might do that on the off hours, but it´s not the focus of the guild and even if people do it, it´s understood that the act of doing content together is not RP. There´s what we do Out Of Character (OOC) and what we do In Character (IC).
OOC Syelia has ran level 8 Delves, killed Ansurek and Gallywix, and got 1600 rating fighting random people in arenas and Horde in BGs. IC, she has been with Kirin Tor for a year, has not fought the Horde and instead of epically beating up nerubian queen spent the time after Dalaran fell riddled with survivor guilt (and not so epically beating up random nerubians).
The reason why I´m saying all this is to make sure you know what you´re getting into and don´t end up in a situation where you join the guild filled with excitement because you´re going to play the game with your guildies and get more connected to the world of this game by pretending that it´s not you behind the computer beating up a fat goblin, but rather your orc rogue who really likes to stab rich fat goblins.
Instead you will be put into a community where you will get to play part of a single character in a story that is actively being created by the other people in the community. I´d say it´s far better than the first option, but I´m still writing all this as a warning so you don´t enter the community with completely wrong expectations of what the activity you´re asking about is.