Anne,
I got this response from a member of one of my groups when I announced
the switch to groups.io . "Do you know that Group.io is a domain from
Indian Ocean? Aren't there any USA or Canada or England supported
domains that we can use?"
I'd say that no, you couldn't choose another domain except by choosing a different service.
Then if you want you can reassure him/her that the domain is administered by a U.K. company and the service itself is run by a U.S. resident (Mark) and hosted by U.S. companies (Linode and Amazon).
Shal
Shal Farley