AD Group
Groups are mainly defined to for assigning permission to shared folders. You can define security groups. instead of applying security for individual users, you can use group to include multiple users. A group can contain multiple users.
Organization Unit (OU)
Organizational units are Active Directory containers into which you can place users, groups, computers, and other organizational units. organizational units can contain other organizational units. A hierarchy of containers can be extended as necessary to model your organization's hierarchy within a domain.
There will be errors(diagnostic code 0x00040503 & 0x00040504) if Splashtop Center fails to connect to AD server or recognize AD Group or OU.