Generalize multipart polygons

Hi all,

I have a layer with multiple polygons sharing a specific attribute (Area_ID), and they are spread with no geometry among them. 

Then, I would like to know the way to identify the area that could be filled considering the vertex points that can make up a border.

Finally, the need is to merge all of the multipart polygons into one, without moving those multipart from the coordinates they are now, just filling the space between the polygons with the same attribute in common.

Any thouhts?

Thanks a lot!