Request.QueryString
この値をasp:SqlDataSource
?に渡す必要があります。
<asp:SqlDataSource runat="server"
ID="SqlDataSource2" ConnectionString='<%$ ConnectionStrings:messageexchangeConnectionString %>'
ProviderName='<%$ ConnectionStrings:messageexchangeConnectionString.ProviderName %>'
SelectCommand='SELECT distribution_address FROM exchange_distribution_groups ex WHERE (organisation_id = 'Request.QueryString["organisation"]') AND (distribution_address NOT IN (SELECT distribution_address FROM exchange_distribution_group_addresses address))'>
</asp:SqlDataSource>