To add/remove mobile users you have to log in to the website app.insitusales.com and then click on the "Field Reps" menu located in the left bar.
Use the "+" to add a new one or click the user you want to edit/delete.
You might want to check the number of licenses you have available. Click on your company profile located in the top bar. You can add new subscriptions or unsubscribe to the current ones.