SimpleTexting allows you to add multiple teammates to your account. Teammates receive access to the list(s) that you give them access to. A teammate will be able to send and receive messages to/from the users on that list. They do not have access to the account's billing information or Apps.
You can add teammates by clicking on the person icon in the upper righthand corner of your screen and choosing Team. You can grant teammates access to all lists, or restrict access to only the lists they need.