The addition of -ler is not dependent on an object. It's optional in either case. Proper grammar dictates that adding -ler/lar is only valid for humans doing an action but in every-day spoken Turkish it's usually omitted. It is also differentiated by a group of people doing something together as a group opposed to a group of people doing something individually. I hope this helps! :)
I think LadyNurington meant that -lar/ler can only be used when a group is doing something together, and not when they are each doing it individually. From my understanding, -lar/ler are not different suffixes, but different forms of the same suffix (depending on what vowel is in the previous syllable).