You can access the Tasks Spreadsheet grouping option from the upper right of the toolbar.
By default, items are grouped in a hierarchic layout to show the project structure. This is a multi-level grouping option where items are grouped under their parent object (ie. project, sequence, shot). Shots are grouped by sequence, and so on.
Other grouping options are Shots/Asset builds and Tasks layout, where the project structure is flattened. It is also possible to group tasks by their attributes, eg. assignee, status, type, a custom attribute. (Some of the attributes are summed at the group level.)