I have a group of ex-students who want to form an alumni association in india. What is the legal process for this? E.g., where and how do we register?
It depends on the purpose of the organization. An applicable law seems to be the Societies Registration Act, 1860 which says
Any seven or more persons associated for any literary, scientific, or charitable purpose, or for any such purpose as is described in section 20 of this Act, may, by subscribing their names to a memorandum of association, and filing the same with Registrar of Joint-stock Companies form themselves into a society under this Act.
Section 20 then adds the list
Charitable societies, the military orphan funds or societies established at the several presidencies of India, societies established for the promotion of science, literature, or the fine arts for instruction, the diffusion of useful knowledge, [the diffusion of political education], the foundation or maintenance of libraries or reading-rooms for general use among the members or open to the public or public museums and galleries of paintings and other works of art, collections of natural history, mechanical and philosophical inventions, instruments, or designs.
It does not seem that an alumni association can straightforwardly be registered as a society. However, it appears that "Society" is a term of art connected with tax exemption. And since you didn't use the word "society", I assume you are not looking for the tax benefit. In that case, the Constitution Article 19 1(a) says that "All citizens shall have the right to form associations or unions; or co-operative societies", which implies that no registration is required.