I do not have the 2013 Professional version of Visio; however, for what you are asking can be done in any version...So, here is my input:
(1) Group all of the shapes that you are wanting to be within your outside dotted line;
(2) Use any shape you want to completely cover the grouped shapes and send that shape to the "back" (or, behind) the grouped shapes;
(3) Remove the background color of your shape (or, recolor to what suits your needs);
(4) Change the outside line of shape to dotted, dashed, or desired line type;
(5) Group the entire objects/shapes together as one.



It has been awhile since I last used Visio in a professional environment, so hope this works for you.

Dear Jim,

thanks a lot.
I really tried but it did not work for me.
(2) is my main problem- how to draw a custom shape or freehand line round the grouped shapes.

I try to post a screenshot to illustrate my needs - hope it works.
There are 4 shapes - 1, 2, 3, 4
I want to have a dotted line arround 1, 2 and 3. Not arround 4.
The best thing for me would be if there was just a possibility to draw a line freehand.

Edit:
Screenshot does not work.
Very easy: 4 rectangles
First line: 1, 2
second line: 3, 4