There were 72.68 crore GSM users at the end of April, the COAI said.
Bharti Airtel, the country's largest operator, added the most users, with 16.5 lakh users joining its network. Its total number of subscribers stood at 20.82 crore, with a market share of 28.41 per cent, at the end of May.
Uninor, controlled by Norwegian firm Telenor, added 13 lakh subscribers to take its overall base to 3.78 crore. Telenor plans to take complete ownership of Uninor by buying Lakshdeep Investments' 26 per cent stake for Rs 780 crore.
Idea Cellular got 11.6 lakh new users to take its total to 13.77 crore. The country's third-largest operator has a market share of 18.79 per cent.
Vodafone India, the second-biggest player in the country, added 9.82 lakh subscribers for a total of 16.82 crore and a market share of 22.95 per cent.
Aircel added 9.42 lakh subscribers to take its total user base to 7.2 crore.
Videocon added 1.27 lakh users and state-run MTNL added 11,702 users. The overall user base of Videocon and MTNL stood at 54.1 lakh and 32.6 lakh, respectively.
GSM refers to Global System for Mobile Communications, a digital cellular technology used for transmitting mobile voice and data services.