1.  **Analyze Existing Group Functionality:**
    *   Examine the existing group management code in `admin.php` and any related files. Understand how groups are currently created, edited, and assigned to athletes.

2.  **Database Schema Modifications (if required):**
    *   Determine if the current database schema can support sub-groups within existing groups or if schema modifications are needed. If modifications are needed, create a database migration script. The script should create a new table to contain subgroups details like subgroup ID, group ID (foreign key), subgroup name (e.g., "Tuesday"), and any relevant metadata.

3.  **UI Design - Admin Interface:**
    *   Modify `admin.php` to include a sub-group management section. This should allow the admin to:
        *   Create new sub-groups within a selected group.
        *   Edit existing sub-group names.
        *   Delete sub-groups.
        *   Assign athletes to specific sub-groups within a group.
    *   The UI should be clear and intuitive, allowing easy navigation and management of sub-groups. Consider using dropdown menus, checkboxes, or drag-and-drop interfaces for athlete assignment.

4.  **Backend Logic - Sub-group Creation, Editing, and Deletion:**
    *   Implement the backend logic to handle the creation, editing, and deletion of sub-groups. This will likely involve creating new PHP functions or modifying existing ones to interact with the database.
    *   Ensure proper validation of input data to prevent errors and maintain data integrity.

5.  **Backend Logic - Athlete Assignment to Sub-groups:**
    *   Implement the backend logic to handle the assignment of athletes to sub-groups.
    *   This will involve updating the database to reflect the athlete's sub-group membership. Ensure that an athlete can only belong to one sub-group within a given group at a time.

6.  **Timetable Integration (timetable.php, coachs-site.php, admin.php):**
    *   Modify the timetable display logic in `timetable.php`, `coachs-site.php`, and `admin.php` to respect sub-group assignments.
    *   When displaying sessions, consider options like:
        *   Filtering the timetable to show only sessions relevant to a specific sub-group.
        *   Highlighting athletes in the timetable who belong to a particular sub-group.
    *   Adapt existing functionality as needed.

7.  **Find Pod Modification (if required):**
    *   Analyze how the find pod algorithm uses existing groups.
    *   Modify the "find pod" algorithm (as referenced in `ChangeLog.md`) to take sub-group assignments into account. The algorithm should prioritize finding pods within the athlete's sub-group.

8.  **CSV Upload/Download Adaption (v8.002.003):**
    *   Adapt the CSV upload/download functionality in `admin.php` to include subgroup assignment information. The CSV format needs to be updated to include a column representing the subgroup.

9.  **User Interface Updates (dashboard.php):**
    *   Update the user interface on `dashboard.php` to accurately display an athlete's group and sub-group affiliations.

10. **Testing:**
    *   Thoroughly test all aspects of the sub-group feature, including:
        *   Creating, editing, and deleting sub-groups.
        *   Assigning athletes to sub-groups.
        *   Displaying timetable data with sub-group filtering.
        *   Running the "find pod" algorithm with sub-group considerations.
        *   CSV upload and download functionality.
    *   Perform user acceptance testing (UAT) with a small group of administrators to gather feedback and identify any usability issues.

11. **Documentation:**
    *   Update the "How-To-Use" landing page (`index.php`) and any other relevant documentation to explain the sub-group feature to administrators.

12. **Deployment:**
    *   Deploy the updated code to the production environment.
    *   Monitor the application closely after deployment to ensure that the sub-group feature is working as expected and to address any issues that may arise.