As per Section 24 of the GST Act, the following categories of persons shall be required to be registered compulsorily irrespective of the threshold limit:
(a) persons making any inter-State taxable supply;
(b) casual taxable persons making taxable supply;
(c) persons who are required to pay tax under reverse charge;
(d) non-resident taxable persons making taxable supply;
(e) an electronic commerce operator for whom the provision of section 9(5) of GST Act apply.
(f) persons who are required to deduct tax under section 51;
(g) Every electronic commerce operator;
(h) persons who supply goods or services or both on behalf of other taxable persons whether as an agent or otherwise;
(i) input service distributor;
(j) persons who supply goods or services or both, other than supplies specified
under sub-section (5) of section 9, through such electronic commerce operator who is required to collect tax at source under section 52;
(k) every person supplying online information and database access or retrieval
services from a place outside India to a person in India, other than a registered person;
(l) such other person or class of persons as may be notified by the Government on the recommendations of the Council.
(ICAI FAQ PUBLICATIONS 06-09-2017 REGISTRATION: FAQ NO. 17)