[CustomPress] Sort posts by custom field

I have a custom post-type “person” that will have a custom field “city”. I am able to sort posts by title, but would also like to sort by “city”, so that for each “city” I could echo the custom field value as a sub-headline ( <h2> ) and then list all post with same “city” – then the next “city” as a sub-headline and the persons with same “city”. Also “city” should be alphabetized as well as the listing of “person”. So the output should be something like this:

<h2>City 1</h2>
- person1_1 (with city1)
- person1_2 (with city1)
<h2>City 2</h2>
- person 2_1 (with city2)
- person 2_2 (with city 2)

Could you provide me with an example of code I can use for this?