How can a veiled girl working in an international organization deal with her irreligious female colleagues? Should she go along with them with the intention of guiding them, or should she avoid them?

We ask Allah to grant you success and to make you among those who invite to Allah with wisdom and good counsel. Your work in mixed environments should be abandoned unless there is a need for it, coupled with the availability of legitimate Islamic controls. As for guiding your female colleagues, it is a religiously مطلوب (required) act. You may employ all legitimate means with them to help them return to their religion, in compliance with the Prophet's (peace be upon him) saying: "Religion is sincere advice," and his saying to Ali (may Allah be pleased with him): "That Allah guides one man through you is better for you than red camels."