To add an administrator go to the Settings tab in your admin dashboard and select Add a school administrator. If you're a district administrator you'll also see the option to Add a district administrator. Once you choose the type of administrator you want to add select the specific school from the drop-down you want to add them to. Then just input their name and school email address and click Add admin.
One thing to note, if the administrator you're adding is new to Remind, they'll need to verify their email address to access their admin dashboard.
To remove an administrator go to the Settings tab in your admin dashboard and hover over the contact you would like to remove. Then simply and select Remove as admin.