Click the Aggregates Link button 
on the diagram toolbar.
Click on the source element.
Drag the link to the destination element.
Drop when the second element is highlighted.
An aggregation relationship denotes that one element contains one or more other elements. This relationship can be drawn between all elements on the diagram, except, from or to, a Collaboration element.
To create an aggregation link:
Click the Aggregates Link button
on the diagram toolbar.
Click on the source element.
Drag the link to the destination element.
Drop when the second element is highlighted.
Related Topics:
Creating Links
Moving Links
Editing Links