From the Available Instructors list at the bottom of the Instructors page in your Instructor Portal, click the Add button to the right of the instructor you would like to add to your SimUText. They will then appear in the Assigned Instructor list at the top of the page. Their User Type will determine what functions within the Instructor Portal they can access.
If you need to add an instructor who is not in the list of Available Instructors, you can create an instructor account for them as explained here.
If you grouped your students into Course Sections, after you have added your co-instructors and TAs, you can link instructors to specific sections. Click the Pencil (Edit) button for that instructor in the "Section Access" column and check off the section(s) to which the instructor should be attached.
To remove an instructor from your SimUText, click on the Remove button next to their name in the Assigned Instructor window and confirm your choice when prompted.