The term "raid" almost automatically signifies more than a single group (otherwise people call it a "group"
)
These are normally organised by guilds, by raiding organisations at higher level, or occasionally on a pick-up basis.
There are things you could raid with a couple of groups at 18 and have fun. Unfortunately a lot of the previously untouched places are now full of 65's twinking alts as part of the Festival of Greed.
Runneyeye Citadel would be a decent choice and I haven't seen a fabled anything in there yet. Remember that if it does go horribly wrong you are all low enough to get a necro with the cheap (2pp) coffin to summon.