No, you've got that exactly backwards. Parliament is the representation. Because of proportional representation, parliament represents all groups in the country, and not merely the majorities in each district.

Exactly.

What? Parliaments obviously cannot represent every group within any given country.

For example, non-residents, tourists, diplomats, etc..

You're right. Parliament represents only the entire electorate. And only really those opinions that are shared by enough people to be worth an entire seat. Still, it's far better representation than you get in a district system, where only local majorities of the electorate in each district are represented.

So then in a country with a weak executive, who collectively represents the interests of non-residents, tourists, diplomats, etc.?

Because each group certainly has some real interests in common.