Nobody. What is now known as the European Union was not founded until 1957. The name European Union was not actually given to this organisation until 1992.
France joined the EEC in 1957 (began functions in 1958), and was a signatory to the Maastricht Treaty that established the EU in 1993.
Actually, it was in 2004. The following countries joined the EU.CyprusCzech RepublicHungaryEstoniaLatviaLithuaniaMaltaPolandSloveniaSlovakia
Hungary joined the EU in 2004, but has not yet joined the Euro currency.
No countries joined the EU in 2008. 10 countries joined in 2004. See the related question below.
Bulgaria has not yet joined the Euro zone, it joined the EU in January 2007.
Sweden joined the EU in the year of 1995.
No new country joined the EU in 1999.
Finland joined the EU in the 1995.
On the 1st May 2004 Hungary joined the EU (European Union)
Malta joined the European Union on the 1st of May in 2004.
The United Kingdom joined the Common Market in 1973.
Croatia joined the EU on July 1st, 2013.