What group received voting rights under the 26th Amendment?

It depends on the country to which you are referring.

In Australia, despite having been populated by Aborigines long before the Europeans came, the Aborigines were the last to gain the right to vote. Aboriginal people became Australian citizens in 1947, when a separate Australian citizenship was created for the first time. Prior to this, all Australians were "British subjects". Aboriginal people gained the vote in Commonwealth territories in 1965, and earlier in different states, according to various state laws.

In the United States, the Twenty-sixth Amendment granted suffrage to people between the ages of 18 and 21. Prior to the amendment's ratification in 1971, voters were required to be at least 21 years of age.