Of peoples that are less than 0.1% Christian (of any kind), there are 4,636 distinct groups. Together, they comprise over 1.7 billion people. Among them there are fewer than 180,000 Christians.
Of these peoples, the 10 largest by population include:
I will grant that this list might be seen by some to be problematic. It originates from Joshua Project and not everyone would label these groups in this way, nor break groups out by caste as JP does. Further, several of these peoples are now lightly (and some even heavily) engaged.
Set all that nuance and messiness aside. The fact is, these are huge groups of people - the Bengalis alone are nearly as large in number as the entire population of the United States. Anything and everything that can be done to reach out to these groups could significantly change their peoples, their nations, and in fact the world.