Jira Service Desk uses global and project permissions to set who can access portals and send requests to service desks. Global customer permissions decide how customers access the service desk. For example, they can decide whether customers can create their own accounts and if they can access the help center without logging in. Project customer permissions decide who can access the service desk. For example, they can decide who can access the portal and who customers can share requests with.
You need to be a Jira administrator to change global customer permissions, and both Jira administrators and project administrators can change project customer permissions.