As others above me have stated, most chatting just goes on in local, or the white colored text chat.
If you're looking for more people to chat with, I'd stick to The Grounds, specifically around the Lonely Cave portal. That's where people tend to gather to recruit, talk about random things, or just have some fun. I've also noticed quite a few people in Cherika Valley, Cape of Distant Worlds, and Seaside Grove.
There's lots of big groups around right now, I think! It depends on what sort of group you're talking about. I once again suggest looking in The Grounds near the Lonely Cave portal for recruiters or just bunches of people. c: