Sforno on Genesis 25:8/ספורנו על בראשית כ״ה:ח׳

1ושבע; he had achieved all he had set out to achieve and had wished for.

2ויאסף אל עמיו, he was attached to the bundle of souls who are part of the life after death, all of whom the righteous of the various generations who were like him in lifestyle, [as in those regions biological relationships are irrelevant, and the term “his people” is applied to other common denominators than on earth. Ed.] The reason why the Torah employs the term עמיו his peoples” in the plural is that there are all kinds of different spiritual levels among the righteous souls, not all attained the same level of righteousness while on earth although all of them share the experience of enjoying eternal life. (compare statements in Baba Batra 75 on that subject.)