This example demonstrates how GroupByExpressions can be used in Telerik RadGrid:

• Creating group-by expressions, testing expressions syntax:
• Expression syntax: fieldname[ alias]|aggregate(fieldname)[ alias][, ...] Group By fieldname[ sort][, ...]
      Where:
          fieldname: name of any field from the DataSource
          alias: alias string. This cannot contain blanks or other reserved symbols like ',', '.' etc.
          aggregate: any of - min, max, sum, count, last, first etc (the same as in GridAggregateFunction enumeration)
          sort: acs or desc - the sort order of the grouped items
      Example: Country, City, count(Country) Items, ContactName Group By Country, City desc  
• Displaying additional information in each group header - like aggregate fields
• Dynamically add a "SubTotal" to each group, handling grid's grouping events

Additional info concerning the grouping feature of the control you can find in the Grouping section from the online help:
Declarative group-by expressions
Programmatic group-by expressions
Customizing GridGroupHeaderItem
Performing calculations in group header