You can do it in SQL in the dataset, or an expression in the report itself, both using substring and charindex/indexof:

SQL:

SUBSTRING(group_name, CHARINDEX('_', group_name), CHARINDEX('-', group_name) - CHARINDEX('_', group_name))

Expression:

=Substring(Fields!group_name.Value, Fields!group_name.IndexOf('_'), Fields!group_name.IndexOf('-') - Fields!group_name.IndexOf('_'))

Note both are untested & you might need to put in something to handle cases where _ or - don't appear.

Cheers

Gaz