If you need to add or remove either users or roles from a certificate, go to the ‘Security Policy’ page (Start Page > Admin > Security Policy) and find the certificate you are looking to add/remove users from, click on it to bring up the policy details.
You will find to the left hand side a list of ‘All Users, Roles and Networks’ and to the right ‘Permitted Users, Roles and Networks’, those to the right are already members of this policy.
To add a user or role simply click on the name in the left hand list at which point it will become listed in the right hand pane (If the user or role is already a member of this certificate an error will be shown explaining this). To remove a user or role simply click on the name on the right hand list.