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Explanation: This would be the usual expression in US English. In UK English one would say "through their lawyers".
"An individual, through their attorney, files for a guardian to be appointed for an adult person when they believe that person is not mentally capable of ..."
"03R1046, Claim of Julian and Magalene James through their attorney, ... 03R1052, Claim of Gregg Ekberfg Patricia Hatter through their attorney, ..."