If you have an temporary staff member standing in at your school, who already has a login to Safeguard Record Manager at their school, you can't just give them access to your school's information. You need to create an email account at your school to be able to add them to your Safeguard Record Manager.
Sally Smith already has a login at her school with an email of sally.smith@school_1.com.
Sally is the temporary acting head at school 2. You can't add Sally with her school's email. You need to create a new email account for Sally, for example, sally.smith@school_2.com and use that to add her to your Safeguard Record Manager.