We wanted to offer more control over who you can chat with in Google Plus. So we’ve made a change that requires you to explicitly invite people for them to appear in your chat list. For Gmail, orkut, and iGoogle users, we’ll continue to include the people you can already chat with in those properties.
Please note that all users in your chat list will have access to your email address. We are working hard to remove this restriction so that you can invite your circles more easily in the coming months.