Thank you for your post into the forum.
To select a group of objects you can use the 'Drag and zoom' tool.
From the View Menu ->Drag and zoom. Click the Left Mouse button and highlight the area you use to investigate further. On releasing the mouse button the diagram zooms in to that area you have highlighted.
Another control you may useful is the connected objects control. Edit Menu ->Select ->Connected Objects. If you click on an object and use the Connected Objects Tool, the control will highlight to you the connected objects.
I hope the above answers your question.
Product Support Engineer
Red Gate Software Ltd