It's a bit complicated. If the verb is passive, the past participle agrees with the subject: a woman would say "Sono vista" and a man "Sono visto", a woman would say "Siamo viste", if all the people of the group she's talking about are females, or "Siamo visti", if there are males and females or the context is more general.
If the verb is active, in most cases the past participle is in the masculine singular form: "Ho visto le donne", regardless of the gender of the speaker. The form "Ho viste le donne", where the past participle agrees with the object, exists, but nowadays sounds literary and a bit poetic, so you should avoid it. If there is an indirect object pronoun before the verb, the past participle can agree with the gender of the person(s) to whom it refers: a man would say "Mi ha visto", but a woman could say "Mi ha vista" or "Mi ha visto", because of that "mi", both work. If you construct the sentence in another way, that's no longer they case: "Lui/lei ha visto me", regardless of the gender of the speaker.