I really want to answer your question but I am now getting an error "There are no workflows currently deployed in the repository, press Cancel to exit the wizard."
Anyhow… usually when the user gets to choose, they can enter the first three letters of a person's name and then a list comes up for them to choose from. This didn't narrow their search enough? Or did you code it such that they didn't get a search box?
I'd suggest to look at parallel_group_review.xml… this allows the person to select a group. Once you figure out how this works the next step would be to automatically grab the results of this, as if the user selected a group.
HTH