Forgive me if I'm missing something, but why would you want to do that?
Wouldn't that mean that when a property is sold, the Passive Losses are NOT released until all of the grouped properties are sold? Is that what you really want? That is usually the opposite of what most people want.
Other than that, I don't see what grouping them would accomplish. Again, maybe I am just missing something, but my first thought is that is a bad idea. Generally, grouping is done in order for a Real Estate Professional to get Material Participation. Grouping for a non-Real Estate Professional seems to be the opposite of what most people would want.
As for your actual question, if you really want to group them, you need an attached statement indicating the grouping. You would probably type it up in a word processing program and attach the PDF to the tax return. But as I mentioned above, my first thought is that is NOT what you want to be doing.