I think this will help you..
• You can’t include high-volume community users in:
◊ Personal groups or public groups.
◊ Sharing rules.
• High-volume community users can’t manually share records they own or have access to.
• You can’t transfer cases from non-high-volume community users to high-volume community users.
• High-volume community users can’t own accounts.
• You can’t add case teams to cases owned by high-volume community users.
Setting Up Your First Community Granting High-Volume Community Users Access to Records
Characteristics of High-volume community users:
• Are contacts enabled to access a community.
• Are assigned to the Customer Community, High Volume Customer Portal, or Authenticated Website license.
• Only share the records they own with Salesforce users in sharing groups.