Text messages are sent through the messages feature. When you select Messages in the Hub Navigation, you will be presented with two options (if hub permissions allow): Announcement and Discussion.
Text Messages are only available through announcements because they are supposed to be short and sweet. When you create an announcement, you can send it as an email and you can copy the first 160 characters in the email to a text. In order for people to receive texts they need to opt in by going to Your Profile, click on Edit Profile, then under Phone Numbers enter a mobile phone number and service provider. Then check the box to allow texts to be sent from your organization to your mobile phone.
MemberHub texting is an email to sms system and will not allow for replies. It is for push notifications via text only.