Besides Jews, the religious community listed as observing Yom Kippur is the Samaritans.
Samaritans are a small ethnoreligious community whose traditions developed alongside Judaism in the ancient Levant. They accept the Torah as sacred scripture and observe several biblical festivals, including a day corresponding to Yom Kippur. For this reason, reference works commonly list the holiday as observed by Jews and Samaritans.
Samaritan religious practice is distinct from mainstream Jewish practice. Samaritans regard Mount Gerizim, near Nablus, as the chosen sanctuary of Israel rather than the Temple Mount in Jerusalem. They also use the Samaritan Pentateuch and reject the authority of the Jewish Talmud, so their calendar, theology, and ritual details can differ.
A common mistake is to treat Samaritans as simply another Jewish denomination. They are historically related to the Israelites and share the Torah, but Samaritanism is recognized as a distinct religious tradition. The community is very small today, with members concentrated mainly in Kiryat Luza on Mount Gerizim and in Holon, Israel. Karaite Jews, Mandaeans, and Druze are separate communities and are not the standard answer to this question.