At the 1999 Washington summit, where Hungary, Poland, and the Czech Republic officially joined, NATO also issued new guidelines for membership with individualized "Membership Action Plans" for Albania, Bulgaria, Estonia, Latvia, Lithuania, North Macedonia, Romania, Slovakia, and Slovenia. In May 2000, these countries joined with Croatia to form the Vilnius Group in order to cooperate and lobby for common NATO membership, and by the 2002 Prague summit seven were invited for membership, which took place at the 2004 Istanbul summit. Russia was particularly upset with the addition of the three Baltic states, the first countries that were part of the Soviet Union to join NATO. Slovenia had held a referendum on NATO the previous year, with 66% approving of membership. A 2006 study in the journal "Security Studies" argued that the NATO enlargements in 1999 and 2004 contributed to democratic consolidation in Central and Eastern Europe.