Chapter III, Article 14 of the Constitution adopted in 1978, allows for freedom of Association along with freedoms of speech, assembly, occupation, and movement. Section 1.c-g covers the circumstances of freedom of association.

CIA World Factbook. Sri Lanka. https://www.cia.gov/the-world-factbook/countries/sri-lanka/#government

Constitute Project. Constitution of Sri Lanka 1978. https://www.constituteproject.org/constitution/Sri_Lanka_2015?lang=en