Patchworks contains a handy tool to define groups. It is the Majority Tool, and it is accessible from the Application Chooser. Given a set of polygons that define zones, and a set of polygons that define blocks, the Majority Tool will identify which zone each block predominantly belongs to.

There are two versions of the Majority Tool. The vector version can be used when a vector overlay of blocks and zones has already been performed. The raster version can be used if the blocks and zones are in two different files, and it performs an 'on the fly' point overlay operation. Regardless of the version being used, the Majority Tool will take a census of the zones that each block overlaps. The output file is a simple .csv file that contains two fields, BLOCK and GROUP. The zone that overlaps the majority of the block is reported as the GROUP that the BLOCK belongs to.

Further information about the Majority Tool can be found in Compute Zone Majority (raster) or Compute Zone Majority (vector).