If you have a list of data that you want to group and summarize, you can do this easily in the Prepare room of Lumira. We do not have to go back to the data source to create the new groups nor use Pivot tables like in Excel. Once we define the Groups we can also use it for building Hierarchies !
There are two methods to Group,
1) by Selection
2) by Range
Both these options are available from the Data Actions panel which appears after you select a dimension. Located on the right side of the application and sometimes you would need to scroll to the end to find it 
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1) Group by Selection
Used when the values are discrete in nature or if there are less values that need to be grouped.
2) Group by Range
Used when the values are continuous in nature or if there are many values that need to be grouped.
Group by Range can used for Date fields as shown above and for Numeric fields as shown below,
In both these cases, there will be a new column that would be created with the new groups.
For detailed instructions to Group data, please refer to the link Create groups: SAP Lumira 1.28 - YouTube