How are 'etre' verbs conjugated differently in the passé composé?

As with AVOIR, the passé composé requires an auxillary and a past participle. However, with 'etre' verbs, aside from the obvious change in auxillary, there are irregularities in the passé composé, and we must add agreements. I'll conjugate an example step by step: my parents went out last night went out= sortir in french, which is MRS VANDERTRAMP and requires ETRE 1- AUXILLARY- like with avoir, we must conjugate the auxillary. My parents is 3rd person plural, therefore we use 'sont' 2.) PAST PARTICIPLE- unlike avoir, there are more irregular verbs, so we should always check. The past participle of sortir is sorti With avoir, we'd have our sentence finished by now- but we cannot say 'mes parents sont sorti hier soir'; with ETRE verbs, we must add agreements onto our past participle, an e for sing. feminine, an s for a plural, or both for feminine plural. As 'my parents' are masculine plural, we only add an s, making the sentence complete.  'mes parents sont sortis hier soir'