UIC, the worldwide organisation for cooperation for railway companies, publishes a large number of ERTMS documents through the so-called "ERTMS platform". These are made available on their dedicated website section.
The website of CER, the Community of European Railways, contains some information on ERTMS.
EIM, the European Rail Infrastructure Managers, occasionally publishes information on ERTMS.
The ERTMS Users’ Group is a European Economic Interest Group (EEIG) formed by a group of several railway administration, and participates to the writing of the ERTMS specifications.