The format of the MEMBERGROUP function is
A object known to be a member of an ObjectGroup, but not known to be any specific member, inherits the properties of all the members of the ObjectGroup (but not the properties of the ObjectGroup) through the MEMBERGROUP operator. Other machinery may set up connections to a subset of the ObjectGroup, or to an individual member (removing the connection to the ObjectGroup in the process), in which case inheritance passes directly from the element in the ObjectGroup to the directly connected object.
Pin 1 controls whether the operator is active. Pin1 can be linked directly to the environment, or be controlled through some logic.
MEMBERGROUP is inverted compared to INVOCATIONS, in that many parents can come to one obect through it.