Messaging capability between users on ArcGIS Online, even if not listed in a group, is essential. The messaging tool does not necessarily have to be an instant messaging tool but rather similar to an e-mail through the ArcGIS Online interface. It would also be essential that members recieving new messages recieve an e-mail.
My suggestion comes after a colleague created a private group and told me I had access to it. Upon signing in I saw this was not the case however the only way I can contact my colleague is via our normal e-mail where my colleague won't have access to my ArcGIS Online account information.
Messages would also be essential to connect with colleagues that you may not have e-mail addresses for which holds especially true during emergency situations. Thank you for your consideration.