The index (ethIfIndex) in the ethIfTable of the interface
group that this interface belongs to. The value -1 denotes
that the interface does not belong to an interface group.
Note that the interface can only belong to an interface
group on the same device as the interface. The interface
can only belong to an interface group type of object,
meaning that the object can only be set to an index of an
object that exist in the ethIfgTable or where the value of
ethIfType is `bETSGroup'.
Setting this to the index of the interface group will add
the interface to the interface group, making it a member of
that interface group. If this is the first interface added
to the interface group, then the interface group will
inherit values on some of its object from this interface.
If this is an additional interface added to the interface
group, then this interface will inherit the same values from
the interface group.