I welcome to WrA there are a number of good guilds and good peoples here. If looking for a smaller but still active guild I suggest Chu Chu Noodle Co. We are more of a social guild, but we tend to do a lot of different things together. More people tend to be on in the evening hours for us but people do pop on at random throughout the day. I might just be a simple old cow, but our leader is wonderful.
I suggest looking at The Horde Guild Directory(Seeks Guilds!). This will help you get a better understanding of WrA’s guild culture and who to find. Big guilds like Life, Epoch, and Trivial Pursuit are some of the big time raiders on Horde side.
I’m unable to gauge what you’re looking for in RP, so I would suggest starting with more social guilds, like Firebrand and Gigavolt. Valley of Honor is also ripe with many other guilds that are too numerous to list.
Alternatively, if you’re looking for a mix of both, I suggest taking a look at The Vol’kar Legion. We combine a healthy mix of RP, PvP, and PvE. We have pushed and achieved every AoTC since Archimonde (WoD) (sans KJ because he’s dumb) and we’ve tried placing our big toe(s) into mythic but have never successfully done it.