Well, for your "Seller" node you could use a simple rectangular node from the "Shape Nodes" palette section and edge creation (the formal term for "attaching message flow arrows") would work as usual.
That said, for all types of groups nodes (i.e. groups, pools, tables, and swimlanes) edge creation works by starting the mouse drag gesture inside the node but near its border - specifically within 10 space units of its border. (Space units means its 10 pixel at zoom 1.0, 20 pixel at zoom 2.0, 30 pixel at zoom 3.0, etc.) Similarly, to end an edge at a group node, you need to release/click the mouse inside the node but within 10 space units of its border.
Edge creation works slightly differently for group nodes to be able to differentiate an edge creation gesture from e.g. a mouse drag gesture that should start marquee selection inside the group.