How do you join a Native American tribe?

Native American tribes are just that Native American - you are born with Native heritage. You just can't "join" a tribe as it is an ancestry.

Answer

A Native American Tribe is a legally defined group. The Bureau of Indian Affairs, under the Department of the Interior keeps track of all American Indian tribes. To join a tribe, you must meet federal and, in some cases, tribal criteria. This includes proof of tribal descent, so you will have to do some ancestry searching.