An admin group is a set of machines which are sharing some config files or parts of some config files. In Linuxconf parlance, they are sharing some subsystems. The concept of a subsystem is the same as that used for the "system profile versioning" feature.
A subsystem represents a set of logically tied configuration files. In some cases, a subsystem is made of only parts of a configuration file.
Seen differently, all configuration files known to Linuxconf belongs to at least one subsystem. In some cases, a configuration is split logically into more than one subsystem. This is the case for /etc/fstab which belongs to the hardware and the netclient subsystems.
This ability to logically distribute one configuration file into various subsystems is the key to sharing. For example, many machines on a net may share the NFS client part of /etc/fstab but can't share the rest as it is hardware dependent (partition layout, swap partitions, etc).