If the objects in the custom library are in the group/lists then the created instances will be added when you drop a copy of the collection/cell into the model (hit reset to check the lists entries). Note that if they are in the list and not in the model - this is also likely to be a problem.
However if you create a new model or load the library, then the objects do not know in which groups they belong. This is because FlexSim objects are not transportable - in that not self-contained and rely on other objects to define their behaviour/ properties - in this case the coupling to Group object. In other cases it's the membership of a Timetable, MTBF/MTTR, or the need for specific parameters in the parameters table. etc...
One way to get around this oversight is to list which groups the object is a member as a label on the object and then use the onDrop or onCreate behavior to reinstate both the group, if it does not already exist, and the object's membership.
Another way is to just assert the membership during the reset based on the label that lists groups of which it is a member.