The role "security_admin" has elevated privileges. For example, if a user has the "security_admin" role, he or she will need to elevate roles to "security_admin" prior to making configuration changes that require that role. If elevate role is not performed, the platform will not allow you to perform functions related to that role.
Hence, a user must click "Elevate Roles" and select the role "security_admin" and then add new members to the group.
To elevate roles, In Homepage, click on the name of the user on the header and it shows a drop-down menu.
In the menu, select "Elevate Roles", check security_admin, and press OK.