Try
The Condemned Catacombs in Antonica.
It starts at a tent camp on the north coast, just east of Oracle Tower. Talk to the NPC's to get some quests, and bring some money. You'll need to buy a key to get into the zone. It cost about 75'ish silver. There is also a NPC behind the tents.
Zone access is on a small island under the cliff. Yell "Geronimo!" and jump off the cliff, and swim to the little island. Be ready for agro as soon as you get there.
Zone entrance is a grate on the ground.
I went there with a full grou of mid-20's and didn't get very far. It might be cramped with 12-18 folks.
You could also check out
Vermin Snye and
Crypt of Betrayal. Although VS may be a little under powered for that many people.
Your post didn't mention if you were in a guild. If so, pick up lots of writs before going into VS or CoB.
As an alternative, you could take all those folks and do access quests to EL and Nek.
Also consider doing a few Heritage Quests.