Take a look at how the MightyText PC texting app looks (on Chrome anyway)... You can see all the recipients on the left, and the currently selected session u are texting in to the right. It's a much better interface than JOIN, where you have to go INTO a text session, and can't see the list of recipients along the left anymore. With all the sessions listed on the left, you can easily switch between them. With the JOIN interface, you have to exit one discussion, go back to the list, pick another etc... Not near as good.
Note: your app works better in general! And i really like how you interact with customers. Well done.