Is one who commented on a post in a group that sometimes contains abominations held accountable for sharing that post with others, even if he avoided the unlawful and enjoyed the permissible?

Your continued presence in the group is conditional upon your ability to enjoin what is right and forbid what is wrong. If you are doing so, then there is no harm in you commenting on permissible things and laughing at them. You are not sinful for someone else's comment on something forbidden, but you must forbid the wrong when you see it, to absolve yourself of responsibility, for the Prophet, peace and blessings be upon him, said: "Whoever among you sees an evil, let him change it with his hand; if he cannot, then with his tongue; if he cannot, then with his heart—and that is the weakest of faith."