Step by Step, to summarise all the above.
Make Door Brush. DO NOT turn it into an entity.
Make Origin Brush. (Axle)
Place Origin Brush where you want it to be, but remember that the door will rotate around the center of the Origin Brush.
Select the Origin Brush and the door (Both Highlighted), and hit CTRL+T.
A property box appears, Select "Func_door_rotating" and assign it's properties.
Test it.
If it doesn't move the way you want it to, then click IG (Ignore group) and select the Origin Brush only and move it. Or change the func_door_rotating properties.
Test it.
When they say group, they actually mean Tie both brushes to an entity.