foo() { bar --baz "$@"; frob --baz "$@"; };
find . -print0 | xargs -0 foo

find . -print0 | xargs -0 sh -c 'bar --baz "$@"; frob --baz "$@";' -- 

Shell functions are only known to the shell. External commands like xargs, su and sudo do not recognize shell functions.

Instead, the function contents can be executed in a shell, either through sh -c or by creating a separate shell script as an executable file.

None.