If you're talking country-wise, most are from the US. Dominic is of course from the UK, and Tomas is from a South American country, though I haven't decided which one.
A few side characters are from other countries as well - Bastien, the dance teacher, is French, and Matt's former roommate, Henri, is Canadian.
If we're talking ethnicity or ancestry, I do have a few specified. Off the top of my head:
Connor - Irish and Romanian
Riley - Spanish and Italian
Griffin - white European and Japanese
Alex - Korean
Dominic - Indian
Less specific:
Asher - Biracial with a Black and a white parent
Marco - Latino
Matt - Possibly German or other northern European ancestry
Leo - Family from some place(s) in the eastern Mediterranean region like Greece, Turkey, Albania, or maybe even southern Italy
I don't tend to nail this down unless it becomes relevant to the story. I'm curious if, based on appearance, anyone has other ideas for the backgrounds of certain characters.