Chatango is a free chat room, pretty much all there is too it. You invite people to your chat, and there you can have a private chat room. I think we already have a page there, not sure.
Kiwi IRC, i'm not really experienced with this but as stated in the name, it is a free Internet Relay Chat. I believe you can password it so you can invite people with the pass.
Facebook, you can invite people to group PM's. Just lie about your age and you in :p I think the messenger app is the same.
Alternatively, you can use WhatsApp, where I think you can have big private group pm's.
Or there is Skype, private video and chat I think.
Kik, Snapchat and GoogleDocs all have some forms of group chat.