Am I sinful for not inviting my atheist friend to Islam, and will I be held accountable for that?

If you are the only one who knows the situation of a non-Muslim girl, or if she cannot learn about Islam except through you, and you are capable of inviting her [to Islam], then abandoning her invitation is a sin that requires repentance. However, if there are others who know her situation, or if she can learn about the religion through other means, then you are not sinful for abandoning her invitation, but you lose out on a great virtue, for the Prophet, peace and blessings be upon him, said: "By Allah, that a single man be guided by you is better for you than red camels." This is because calling to Allah, enjoining what is good, and forbidding what is evil is a collective obligation upon the Ummah, and it may become an individual obligation upon a Muslim if no one else is available to perform it.