You can choose to AutoFill shift assignments based on seniority. This takes the most senior helper (oldest begin date) and assigns them according to their availability and help time preferences and attempts to give them up to their maximum hours per week. It then takes the next senior helper and fills in the empty shifts giving the next helper his most preferred schedule, etc. and continues for each helper. Using this method results in your most senior helpers getting their most preferred schedule (based on the help time preferences they have entered) and will ensure your most senior helpers get their max hours per week.
Note that the helpers begin date can be added when you add a helper or at any time when you edit a helper. You can click helpers in the top menu, then click the "Select Columns" link, and view and edit the begin dates directly in the helper grid. Then click the Save button to save all.