There's a lot of good guilds on this server, and you can find a summary of all of them
here. If you're looking for casual raiding the middle group will probably be what you want, although not every guild in that section would consider themselves "casual".
Personally though, as a casual raider myself, I'd recommend taking a look at Anonymous. We use a DKP system and raid every week on Wednesday and Friday, 5pm EST. Wednesday is Plane of Sky night (melee haste belts and some other good items) and Friday is HoT night (get yourself a full suit of
Dragon armor). We also raid other targets on other random nights.
Every raid is 100% optional: you'll never be forced to come. We do have an attendance requirement to get loot though, so if you stop coming for a month or longer (which is totally allowed), you won't be able to earn loot when you come back ... for your first couple raids back (and you can still earn DKP on those raids). Also, while we're not the biggest guild on the server, we're in the Aegis alliance. This is nice because we're small enough that you can get to know everyone, big enough to raid a lot on our own, and for the even bigger stuff we'd normally miss out on we get to raid with Aegis.
You don't need to know how to raid at all (we'll teach; eg. every week we re-explain how to do Sky), but we do use Discord and expect people to listen (you don't have to talk). If all that sounds appealing check us out; we'd love to have you.