Well, as she said, you can send 1 PM and have it go to multiple people. They in turn can reply to everything by hitting "reply all"
But if you're looking for an actual way to chat there are several sites you can go through. Instant messengers as well. Theres skype, YIM and the sites like tinychat where you can have a private chatroom with only you and those with the password.
I used tinychat quite a lot before I got a skype. Some people have 2 skype accounts. One for their normal life and one specifically for discussing RP's and keeping up with partners.