Part of Schengen, part of the European Free Trade Association, signed a lot of bilateral treaties and enacted a lot of EU legislation, contributed to EU projects.
The Swiss electorate voted against becoming part of the EU, but the EU's a huge market so Switzerland is quite integrated with the EU while still being independent.
Norway isn't part of the EU either, but they're in a similar position as Switzerland, which is probably why they're also part of this proposal.