Loopindex is kind of iteration command repeated based on condition input. In your case loopindex iterates through all family member.For instance if you have 5 family member, loopindex repeats command 5 Times.
Meanwhile PickedCount Will show up as auto complete if you type object name followed by dot then add PickedCount in expression field. Expression Seems daunting at first but very handy in return.
Here much much simple example summoning family member without randomness:
dropbox.com/scl/fi/7kxctgsdlp8nvh46xdev8/Loop_through_family.c3p
Next example just incorporating loopindex to create grid:
dropbox.com/s/i58n1yqjw0ccqw0/loop%26spawn.capx