With avoir as the auxiliary, the past participle normally doesn't change at all: elle a mangé, ils ont fini, regardless of the subject's gender or number. The one exception is when a direct object comes before the verb — in that case, the participle agrees with that preceding direct object instead of with the subject.
This happens most often with a direct object pronoun: j'ai vu la voiture becomes je l'ai vue (I saw it — la voiture is feminine, so vue takes an -e) because l' (standing in for la voiture) comes before ai vue. It also happens with que in a relative clause: la voiture que j'ai vue. This rule affects spelling far more than pronunciation and is a common source of small agreement errors even among fluent writers.