The country must be in Europe, has to apply for its membership. After it is approved, the process of negotiation and accession talk will start. Its a long process and the candidate member should fulfill the criteria.
Germany did not become the EU. Germany is one of 28 countries that is a member of the EU. The EU as we now know it came into existence on the 1st of November 1993, when it had 15 members.
It became an EU member on 1st January, 2007.
Denmark did not leave the EU. It is still a member. It does not use the Euro, but that does not mean it is not in the EU. Some other EU countries do not use the Euro. Only 19 of the 28 member countries use it.
The UK was a founding member of the EU.
Croatia will become a member of the EU on July 1, 2013.
As Bulgaria is a member of EU, Bulgarians freely move and pass though EU countries
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There is no 7th member of the EU. What was later to become the European Union was founded by 6 countries in 1957. On the 1st of January 1973, the membership jumped directly to 9 when 3 new countries joined. Those 3 countries were Ireland, the United Kingdom and Denmark. None can really be said to be the 7th member, as they all joined at the same time.
Croatia joined the EU on July 1st, 2013.
Bavaria is a region of Germany. Germany is a member of the EU. It is countries, not regions that are described as being members. So Bavaria is in the EU, but it would not be described as being a member in its own right.