Supreme Court has held in Labishwar Manjhi  v. Pran Manjhi,(2000) 8 SCC 587, that if members of Tribes are Hinduised, the provisions of the HMA, 1955 would be applicable.

2) the manner in which the marriage had been conducted in the present matter and the customs being followed by the parties show that as in the case of Hindus, the marriage is conducted in front of the fire. The Hindu customary marriage involves the ceremony of Saptapadi which has also been performed in the present case.

3)members of a tribe voluntarily choose to follow Hindu customs, traditions and rites they cannot be kept out of the purview of the provisions of the HMA, 1955.

No, Tribal people not comes under Hindu law. If their marriage needs to come under Hindu law then kindly perform marriage under special marriage act.

Even though Hindu Marriage Act not applicable to tribal people, but if they follow Hindu Customs voluntarily, then the Hindu Marriage Act is applicable.

It has been clearly stipulated in the Act that the provisions of the Act are not applicable to members of the Scheduled Tribe unless there is a notification in the Official Gazette, otherwise directs..

In the Constitution of India, or in any other enactment of the Parliament the term ‘hindu’ has not been defined in a comprehensive meaning. During a search one can see that it has not been defined in the Hindu Marriage Act or in any other enactment.

Therefore it is not applicable. 

HMA is applicable to any tribes or tribal communities unless tribe or community is not specified through notification by the President of India under Article 342 to be Scheduled Tribe. That is, if the Tribe to which you belong is not so specified, HMA is applicable to you. 

Dear client Hindu marriage Act does not apply to tribal people.

Yes if both the parties have Accepted Hinduism and married then it's valid 

Yes. It would be applicable if the couple practices Hinduism.

Yes, very much applicable. It is the religion they practice that matters, and not whether they belong to any tribe or not, which is immaterial.