When a verb's auxiliary is être in a compound tense, its past participle behaves like an adjective and agrees with the subject: il est allé (masculine singular, no extra ending), elle est allée (feminine singular, add -e), ils sont allés (masculine plural, add -s), elles sont allées (feminine plural, add -es).
This agreement is often invisible in speech — allé, allée, allés, and allées are all pronounced the same way — but it matters in writing. A mixed-gender plural subject (e.g., "he and she") takes the masculine plural form, the default for any group containing at least one masculine member: Paul et Marie sont partis.