The accounts permitted to sponsor guest users are authenticated by either an external Identity Store or internal Network Access User accounts. You would need to look at how your Sponsor Group memberships are configured to determine who can Sponsor what types of Guest accounts.
As per the screenshot below, for example, this Sponsor Group is used for members of my 'ISE Admins' AD group as well as any internal user accounts that are members of the internal User Identity Group 'ALL_ACCOUNTS'
