I agree with Ivo on the main point: No. --However, I don't know the history of Hinduism (and the possible interrelations of tribal folk in India with Hindu peoples) the way he does. But my reading of the tribal cultures in anthropological works certainly makes them seem isolated as to economy, religion, and culture (see for example the Nayars) and they seem to have been living in inaccessible areas--when compared to the agricultural Hindu population. Their religious practices aren't Hindu as far as my reading goes, and that would decisively exclude them. Please let me know what you learn, as this is one of the areas I am very interested in. -- AP
Yes in india tribal people are mostly hindus. Hinduism is a feminine religion. The origin dates back to vedic era in the banks of indus. But slowly the religion progressed from its indigenous population towards the forest people. As hinduism believes in pluralism all the tribal deities were included in Hindu gods. Some tribal mythology took prominent space in Hindu mythology. Slowly the tribal culture fused in the great hindu philosophy. Hindus also started praying many tribal deities. Deities like banadurga, manisha had a big place in Hindu temples. Now in twenty first century many Indian tribal people are considered hindu and for there upliftment in society they are called as scheduled tribe. After missionary infiltration in some tribal area some tribal people have been converted into Christianity. But officially they are included in scheduled tribe group of hindus.
There are tribal people in India that are Hindus, but the majority are not. There are about 9 percent of India's population categorized as tribes, which is more than 100 million people. Many tribals follow traditional tribal religions, they often have syncretized with Hinduism, Buddhism, Islam or Christianity. A concentration of Christian tribal groups is in the North-East, especially in Assam, Nagaland, Mizoram, and Meghalaya. Also Santhal groups in Jharkhand, West Bengal, Bihar, Odisha, Assam are often Christians, but also often follow at the same time their traditional religions.
Recommendation
Christoph von Fürer-Haimendorf Tribes of India: The Struggle for Survival University of California Press 1982
Edgar Thurston Castes and Tribes of Southern India Edgar Thurston Vol 1-7