Chatango is primarily focused on group chats, but you can send private messages as well. Dropinchat.com mentions "chat groups". Are they only for user-to-user messages?
The documentation on dropinchat.com mentions "groups" as a set of users which are allowed to see one another and message back and forth. Our team thinks this use case might be helpful, for example, for a website administrator to limiting communication between members on a given organizational team or individuals in a certain building, etc.