Does everyone have to be using Hub in order to receive and reply to messages?
This article describes if SMS message recipients need to have Hub installed.
No. You are able to send SMS messages from Hub to users who are not using the app, the recipients don't even have to have the same carrier or device as you do. The only requirement is that they have the ability to receive a SMS message.
When a recipient receives an SMS message from Hub it will appear just like any other text. They will also be able to reply without having to download or use Hub.