Suppose your questions are in a group named geometry (before the use of \onecopy):

\element{geometry}{
  \begin{question}{first}
  ...
  \end{question}
}

[... nine other questions ...]

You can shuffle the questions inside this group with \shufflegroup, and then insert the 5 first questions of the shuffled group with \insertgroup (with an optional argument), inside \onecopy:

\shufflegroup{geometry}
\insertgroup[5]{geometry}

As an other example, you can build an exam with 5 questions from geometry and 6 questions from algebra, all shuffled, with the following code:

\cleargroup{all}
\shufflegroup{geometry}\copygroup[5]{geometry}{all}
\shufflegroup{algebra}\copygroup[6]{algebra}{all}
\shufflegroup{all}
\insertgroup{all}

See Groups of questions section in (a recent version of) AMC manual.