Groups are useful for categorizing data under common labels. Groups simply add another layer of labels/headers over existing data and can only be used with dimensions. Examples of grouping would be to correctly categorize data, for example, NY and New York all under a common header of New York, or to create regional groups per manager depending on their territories. 

Creating groups is similar to creating sets. You can create a group directly from the visual by selecting the chart and then clicking on the paper clip icon for grouping:

Once you group the items, you will see Market (group) appear in the Dimensions shelf. Right-click and select Edit Group. You will see the menu as follows: 

Here, you can group other members together, ungroup current members, and rename the groups as well as choose to include an Other group. As an example, let's combine Africa and EMEA by selecting both, clicking on Group, and then naming it EMEA++. Also, check the Include 'Other' button and click OK. Now, bring Market on the Rows shelf after Market(group). You will see the graph shown here:

As you can see, the various Market types have been grouped together under the labels we chose in Market(group).

Another way to perform grouping is to select the Dimension the elements of which we want to group, right-click and select Create | Group. This will open the same menu as you saw when clicking on Edit Group before.