The rules on accession of a new Member State are set out in Article 49 TEU, as follows:

Any European State which respects the values referred to in Article 2 and is committed to promoting them may apply to become a member of the Union. The European Parliament and national Parliaments shall be notified of this application. The applicant State shall address its application to the Council, which shall act unanimously after consulting the Commission and after receiving the assent of the European Parliament, which shall act by an absolute majority of its component members. The conditions of admission and the adjustments to the Treaties on which the Union is founded, which such admission entails, shall be the subject of an agreement between the Member States and the applicant State. This agreement shall be submitted for ratification by all the contracting States in accordance with their respective constitutional requirements. The conditions of eligibility agreed upon by the European Council shall be taken into account.

So aside from a laundry-list of requirements the EU has to act “unanimously” when accepting a new member.

Source: EU Law Analysis: EU Referendum Briefing 1: Can the UK control the EU’s future if it stays a member?