SimpleTexting allows you to add multiple sub-users to your account. Sub-users receive access to the list(s) that you give them access to. A sub-user 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 users by clicking on your name in the top right and choosing Sub-accounts. You can grant sub-users access to all lists, or restrict access to only the lists they need.