If you edit the group and it has been associated with a query, on the Group Information Tab, there will be a section called Associated Query Information with a checkbox, "Periodically rebuild group membership?", and a set of frequencies to choose from.

Note:  The specific time of day cannot be chosen, and the day of the week is based on the last time that the group was rebuilt.  If the rebuild runs overnight where it starts at the end of the day and ends at the beginning of the next day, the day that it will run will be a week (or month, or year) after the date that the rebuild ended.