Hi,
you can do it just as you would place any other control:
<%@ Register TagPrefix="MonoX" TagName="PopularGroup" Src="/MonoX/Samples/WebPartSample/PopularGroups/PopularGroupsList.ascx" %>
and place control to the content place holder:
<
monox:PopularGroup
id
=
"test"
runat
=
"server"
></
monox:PopularGroup
>
Regards