I assume you mean grouping a search results by a managed property such as organisation or location, and these kind of things. That you have a result which displays content grouped by for instance location.
If this is the case and you want to group result from a single query, you have to edit the web part and change the query.
- In the Web Part, click the Web Part Menu --> Edit Web Part.
- In the Web Part tool pane, click Change query. This opens a dialog box.
- In the dialog box, from the Select a query list, select your result.
Click Switch to Advanced Mode. In Advanced Mode, in the Query text field, you’ll see some text. This is the query text that represents your result.
In the Property filter list box, select Show all managed properties, and select your managed property.
- Click Add property filter, and select a second prtoperty (if you need to).
- To test that this query returns the correct results, click Test query.
Reference and tutorial: Configure the query in a Content Search Web Part on a catalog item page
If this is not the case, and you need to merge differnt result sets, then you need to do is Create and display a result block. There you have the option to group search results in different reult blocks.
A result block is several search results that are displayed as a group. In the same manner as you can promote a specific result, you can promote a result block when a specified query condition applies. For example, you can create a result block named Yellow items for all catalog items that have the color yellow. In an Internet business scenario where you have a catalog of electronic products and you want to promote yellow mp3 players, you can create a query rule that fires for all items that are tagged with the term mp3 players, where the action is to display the result block Yellow items.