Answer
1973 (UK, Ireland, Denmark); 1981 (Greece); 1986 (Spain, Portugal); 1995 (Austria, Finland, Sweden); 2004 (10 states); 2007 (Romania, Bulgaria); 2013 (Croatia).
💡From 6 founders to 28, then 27 after the UK left (Brexit, 2020).
EU history & treaties (EUR-Lex / european-union.europa.eu)